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We report a low-threshold and efficient Alexandrite laser that is pumped by only one state-of-the-art single-spatial-mode diode. The pump diode provided 170 mW of output power at 635 nm. In continuous wave (cw) laser experiments, we demonstrated lasing thresholds as low as 13 mW, slope efficiencies as high as 36%, output powers up to 48 mW, and a tuning range extending from 736 nm to 823 nm. Laser slope efficiency, laser output power, fluorescence lifetime, and emission intensity were further shown to decrease monotonically with increasing temperature. Pure cw operation could be obtained under most circumstances. However, self-Q-switching (SQS) was also observed in slightly misaligned laser cavities. During SQS, stable pulses with pulsewidths in the 5-15 μs range and pulse repetition rates in the 10-35 kHz range have been obtained. We believe that this compact and efficient Alexandrite laser system may be an attractive source for several applications.
© 2014 Optical Society of America
1. Introduction
In recent years, progress in diode technology has enabled the production of higher brightness single-mode diodes (SMDs) in the red and near infrared spectral regions. For example, in the red spectral window (630-680 nm), output powers from SMDs increased from around 5 mW level [1] to above 150 mW over the last 20 years [2]. Moreover, the cost per watt of output power has also been steadily decreasing. The progress observed in SMDs has enabled their wide range use in many areas including DVD players, laser pointers, barcode readers, spectroscopy, and DNA sequencing, among others. One other interesting application area of SMDs is diode pumping of solid state lasers. Nearly diffraction limited output beam profile of SMDs makes it possible to construct efficient and low-cost solid-state laser systems. Moreover, SMD pumped solid-state lasers have additional advantages including compactness, reduced cooling requirements, portability, improved electrical efficiency and low laser noise. SMD lasers have been used to pump many solid state gain media to date, including Nd:YAG [3], Nd:Glass [4], Cr:LiCAF [5], Cr:LiSAF [6], Yb:YAG [7], Tm:YAG [8], and Alexandrite [1].
Alexandrite (chromium-doped chrysoberyl, Cr3+:BeAl2O4) is the first tunable laser crystal operated at room temperature [9, 10]. Its wide emission bandwidth around 800 nm enables broadband wavelength tuning of Alexandrite lasers in the 700-855 nm wavelength range [10, 11]. This bandwidth is also wide enough to support sub-10-fs pulses, but so far the shortest demonstrated pulses are in the ps range [12]. Alexandrite crystals can be produced with low passive losses (0.06%/cm [13]), have an intrinsic laser slope efficiency of 65% [13], and have long room-temperature fluorescence lifetimes (260 μs [14]) that facilitate efficient flashlamp pumping. Furthermore negligible concentration quenching of fluorescence [15] and the large thermal conductivity (23 W/ (mK) [14]) of the host enable power scaling. Moreover, its broad absorption band around 600 nm (FWHM ~100 nm) and zero-phonon transition lines at 678.5 and 680.4 nm, lead to efficient direct diode pumping with red diodes (630-685 nm). Alexandride has been one of the most popular tunable lasers sources in the market and has been used for many applications ranging from medicine [16] to differential absorption lidar [17–19].
Interestingly, Alexandrite is also the first broadly tunable solid-state laser that was directly diode pumped [1]. At that time, using two 5 mW SMDs around 680.4 nm [1], Scheps et al achieved lasing with output power less than 1 mW. Three years later, two 250 mW single-emitter multimode diodes around 640 nm were used to pump Alexandrite, and cw output powers of 25 mW and slope efficiencies of 28% were demonstrated [13]. High power diode arrays at 680 nm (10 W [20]) and 635 nm (29 W [21]) have also been used as pump sources, and cw output powers as high as 1.3 W [20] and 6.4 W [21] have been demonstrated. However, low beam quality of the diode arrays results in multimode laser output and limits the obtainable slope efficiencies to below 25% [20, 21]. Recently, 1-W tapered diodes at 678.5 and 680.4 nm have also been used as pump sources, and cw powers up to 200 mW and slope efficiencies up to 38% have been realized [22]. The laser output was multimode with an M2 of around 2.5, and cw powers and slope efficiencies decreased to 110 mW and 25% level, respectively, when the laser is pushed to produce a TEM00 output beam profile [22].
In this study, we present a detailed lasing investigation of a low threshold and efficient Alexandrite laser that is pumped by one state-of-the-art SMD. The simple pumping scheme employed in our study enables the construction of a compact and efficient Alexandrite laser system that is attractive for many applications. A single-mode laser diode providing 170mW of output power at 635 nm was used as the pump source. Compared to earlier SMDs (5 mW in [1]) that were initially used for pumping Alexandrite, the current SMD has 34 times higher brightness. In laser experiments, cw output powers as high as 48 mW, slope efficiencies of 36%, lasing thresholds as low as 13 mW and a wavelength tuning range from 736 nm to 823 nm have been realized. Stable self-Q-switching operation of the Alexandrite laser was also observed, and laser pulses with duration in the 5-15 microsecond range at repetition rates between 10 to 35 kHz were observed. Laser slope efficiency, laser output power, fluorescence lifetime and emission intensity were shown to decrease monotonically with increasing temperature.
The paper is organized as follows: Section 2 introduces the experimental setup and the methods employed in the spectroscopic and laser characterization experiments. In Section 3, we present experimental results on emission spectrum and fluorescence lifetime measurements. Section 4 presents detailed continuous wave lasing results. In Section 5, we summarize the results and provide a general discussion.
2. Experimental
In our experiments, two different Alexandrite crystals with Cr3+ doping levels of 0.13% and 0.2% were used (Synoptics, Inc.). The 0.13% Cr-doped crystal was 8 mm long, 5 mm wide and 3 mm thick. The 0.2% Cr-doped crystal had the same width and thickness but had a length of 10 mm. Both crystals were Brewster/Brewster cut to minimize reflection losses at the pump and lasing wavelength. Moreover, the crystals were cut so that, for TM polarized incident light, the direction of the electric field of the electromagnetic wave inside the crystal is parallel to the crystal b axis. The crystals were mounted with very thin gold foil in a copper holder. The temperature of the copper crystal holder was stabilized by using a controller and can be adjusted in the 25 - 300 °C range.
Emission spectrum and fluorescence lifetime measurements were carried out using a pulsed green source based on the frequency doubled output of a commercial Nd-vanadate laser at 1064 nm. The pulses had a full-width-half-maximum (FWHM) of around 70 ns. A pulse repetition rate of 250 Hz were used in lifetime and emission spectrum measurements, respectively. Pulse energies of around 1 μJ were sufficient to perform the measurements. For each case, the pump beam was focused at the center of the Alexandrite samples to a beam waist of around 25 μm using a converging lens with a focal length of 60 mm. The fluorescence signal was collected with a MgF2 lens with a focal length of 8 cm at 90° with respect to the direction of the excitation beam. For emission spectrum measurements, we used a commercial spectrometer (Ocean Optics, model USB2000-VIS-NIR) which had a resolution of 1.5 nm and a spectral responsivity covering the range from 350 nm to 1000 nm. The time dependent fluorescence decay signal was measured using a fast (1 GHz) Si detector and recorded with a 500 MHz digital sampling oscilloscope. The temperature dependence of the emission spectrum and the fluorescence decay time were measured for crystal holder temperatures ranging from 25 °C to 300 °C. We note here that due to the relatively high thermal conductivity of the Alexandrite crystal, the calculated temperature difference between the crystal holder and the center of the crystal is at most 2-3 °C. Hence, we assumed that the center temperature of the crystal is nearly equal to the holder temperature.
Figure 1 shows a schematic of the Alexandrite cavity that was used in the continuous-wave (cw) laser experiments. A linearly polarized, 635 nm AlGaInP single-mode diode was used to pump the alexandrite crystal. The SMD temperature was stabilized to 20 °C with a thermoelectric cooler. The SMD provided a maximum output power of 170 mW at a diode drive current of 300 mA. The output of the SMD was first collimated using a plano-concave lens with a focal length of 4.5 mm. An anamorphic prism pair with a magnification of 2 was then used to transform the elliptical output beam to a nearly circular profile. The pump beam was focused inside the alexandrite crystal using an anti-reflection coated achromatic doublet with a focal length of 60 mm (L2). An astigmatically compensated 4-mirror x-cavity housed the Alexandrite crystal during the continuous wave laser experiments. The cavity consisted of two curved mirrors (CM1 and CM2, each with a radius of curvature of 75 mm), a highly reflecting flat end mirror (HR) and an output coupler mirror (OC). Cavity high reflectors had reflectivity greater than 99.9% from 750 to 850 nm. The pump mirror (CM1) had an additional anti-reflection band around 650 nm which transmitted more than 95% of the pump beam. Arm lengths of 30 cm and 55 cm resulted in a beam waist of around 25 μm inside the Alexandrite gain medium near the center of the cavity stability region. The laser performance was investigated by varying the crystal holder temperature and by using output couplers with different transmission. In the cw tuning experiments, a quartz birefringent filter was used to vary the laser output wavelength.
Fig. 1 Experimental setup of the single-mode diode pumped continuous-wave Alexandrite laser. SMD: Single-mode diode, BR plate: Birefringent plate for laser wavelength tuning, OC: output coupler.
3. Temperature dependence of the spectroscopic properties
In this section, we first discuss the energy level diagram of the Cr3+ ions in the Alexandrite crystal. We then summarize the results of our spectroscopic measurements which show the temperature dependence of the emission spectrum and fluorescence lifetime of the Alexandrite crystal for temperatures ranging from 25 °C to 300°C.
Figure 2 shows a simplified energy level diagram of the Cr3+ ions in the Alexandrite gain medium for the E//b orientation. Alexandrite possesses two strong and broad absorption bands (each with widths of approximately 100 nm) with absorption peaks located near 410 nm and 590 nm [9]. Transitions from the ground state (4A2) to the vibronically broadened 4T1 and 4T2 states are responsible for these absorption bands. 4T1 and 4T2 states have relatively short lifetimes (for example, the intrinsic lifetime of the 4T2 state is around 6.6 μs) and they decay rapidly to the metastable 2E state, which has a lifetime of around 1.54 ms [14]. The energy difference between the 2E and 4T2 levels is only 800 cm−1 in Alexandrite (a few kT at room temperature). Hence, after optical excitation, a significant amount of the states in the 4T2 level is populated in the thermal quasi-equilibrium; since 2E acts as a storage level. Four-level lasing operation in Alexandrite has been obtained using the transition of excited ions from the 4T2 level to the 4A2 level. So, we will investigate the temperature dependence of the emission spectrum and fluoresce lifetime for the 4T2 and 2E levels to understand the effects of increased temperature on the cw laser parameters. Here, we note once again that the dynamics of the 4T2 level cannot be separated from that of the metastable 2E level, which acts as a reservoir of ions for the 4T2 level.
Fig. 2 A simplified energy level diagram of the Cr+3 ions in the Alexandrite crystal for the E//b orientation [23].
Figure 3 shows the measured variation of the fluorescence lifetime as a function of the crystal temperature for temperatures ranging from 25 °C to 300 °C. The measurement was performed for the 0.13% and 0.2% Cr3+ doped alexandrite crystals, and the measured lifetimes overlap quite well in the whole temperature range. This indicates that, Cr3+ concentration levels in the Alexandrite samples do not play a very significant role in the fluorescence dynamics (at least for these concentration values). This is in good agreement with the previous reports, which reported lack of concentration quenching of fluorescence lifetime in Alexandrite [15]. We measured the room temperature (25 °C) fluorescence lifetime of the alexandrite crystal to be 262 µs and 268 µs for the 0.13% and 0.2% chromium-doped samples, respectively. Considering the experimental error bars ( ± 5 µs), the measured fluorescence lifetimes can be considered identical and they are also in good agreement with the previously reported values in the literature (262 µs in [14]). The lifetime for each sample decreases monotonically, and reaches a value of 55 µs at 300 °C. In this temperature range, a similar decrease in the fluorescence lifetime was also reported in previous studies [14].
Fig. 3 Measured variation of the fluorescence lifetime as a function of the crystal temperature for the 0.13% and 0.2% Cr3+ doped alexandrite crystals.
For transition metal ion-doped laser materials, the Mott equation could be used to model the temperature dependence of the fluorescence lifetime () [24, 25]:
Here, is the spontaneous radiative decay rate, is the temperature-dependent non-radiative decay rate, is the high temperature limit of the non-radiative decay rate, is the activation energy, k is the Boltzmann’s constant and T is the absolute temperature, in degrees Kelvin. Basically, the Mott equation states that, as the temperature (and hence the phonon energies) increases, the rate of phonon interactions/decays increases, and this causes a general decrease of the fluorescence lifetime. In Fig. 3, the solid lines are the least-squares fits to the lifetime data for 0.13% and 0.2% Cr-doped Alexandrite crystals, where we took τR as 1540 μs (low temperature value of the fluorescence lifetime [14]). The best-fit values of and were determined to be 9.51 μs and 717 cm−1, and 9.55 μs and 722 cm−1, for the 0.13% and 0.20% Cr-doped Alexandrite samples, respectively. These values are in good agreement with what has been reported in earlier studies (6.6 μs and 800 cm−1 [14]).We note here that, an increase in the fluorescence lifetime of the 4T2 level was reported in [14] for temperatures up to 70 K (−200 °C), and attributed to a thermal excitation process of ions within the 2E level (this level is actually a doublet, and the upper lying level has a longer lifetime, the details of which can be found in [14]). Above 70 K (−200 °C), the fluorescent lifetime decreases monotonically due to the thermal excitation of higher lying 4T2 level (the lasing level, 4T2 has an intrinsic lifetime of around 10 µs, and excitation to this level actually causes a reduction in the fluorescence lifetime). Basically, due to the interaction of the 4T2 and 2E levels via phonons, excited ions are transferred from the reservoir (2E) to the laser active 4T2 level. The interaction gets stronger at higher temperatures and this in turn reduces the effective fluorescence lifetime of the system. On the other hand, this interaction increases the effective stimulated emission cross section of the lasing level [26]. Hence, in pulsed laser operation, despite the decrease observed in the fluoresce lifetime, by choosing an optimum pump pulse duration (shorter than the fluorescence lifetime), the laser efficiency could actually be improved at elevated temperatures, owing to the increase observed in effective stimulated emission cross section [11].
Figure 4 shows the measured variation in the fluorescence spectrum of the Alexandrite crystal around 800 nm, as a function of the crystal temperature, for temperatures ranging from 25 °C to 300°C. We first note that the emission strength (emitted number of photons at each wavelength) decreases as the crystal temperature is increased. This is mainly due to the decrease in the fluorescent lifetime at increased temperatures (the decrease in the fluorescence lifetime is stronger than the increase in the effective emission cross section). Second, the peak emission wavelength for the 4T2 → 4A2 transition shifts from around 700 nm at room temperature to around 725 nm at 300 °C. A much slower shift is also observed in the peak wavelength of emission resulting from the transition from the 2E (R1/2) level to the ground state (emission from the R lines) [27, 28]. Moreover, note that, with increasing temperature, the strength of the 2E emission decreases sharply. For example, in the emission curve taken at 300 °C, we don’t even see any structure around 680 nm any more. Basically at these temperatures the effective emission cross section of the 4T2 transition is much higher than the 2E level due to the increased phonon dynamics at elevated temperatures.
4. Continuous-wave lasing results
In this section, we present our continuous-wave lasing results obtained with a single-mode diode pumped Alexandrite laser. Figure 5 shows the measured cw laser efficiency of the Alexandrite laser at room temperature using five different output couplers with transmissions ranging from 0.1% to 1.7%. The data were taken using the 0.2% Cr-doped, 10-mm-long Alexandrite crystal, which absorbed 95% of the TM polarized incident pump light. The free running laser wavelength was around (760 ± 10) nm with all the output couplers. The spectral width (FWHM) of the laser output was around 3 nm. The laser output beam had a TEM00 beam profile, with an M2 value better than 1.05. Output powers as high as 48 mW and slope efficiencies as high as 36% were obtained using a 0.5% output coupler. The absorbed pump power level was 158 mW, which corresponds to an optical-to-optical conversion efficiency of 30%. With respect to incident pump power, the optical-to-optical conversion efficiency was 29%. The measured lasing threshold was 28 mW. The measured threshold pump power was as low as 13 mW with the 0.1% output coupler.
Fig. 5 Measured output power variation as a function of the absorbed pump power for the cw alexandrite laser taken with various output couplers (OCs) having transmission values between 0.1% and 1.7%.
Note from Fig. 5 that, the slope efficiencies obtained with 1.2% (31%) and 1.7% (29%) output couplers are lower than what can be achieved with the 0.5% (36%) output coupler. Similar trends have also been reported in cw Alexandrite lasers pumped by tapered diodes at 680 nm [22]. This unexpected decrease of the slope efficiency at increased output coupling values limited the obtained slope efficiencies (36%) and output power levels (48 mW) in our study. A similar effect is also observed in Cr:Colquiriites due to the presence of Auger energy transfer upconversion (known as ETU) [29, 30]. In the literature, we could not come across previous work that identified ETU as a possible transfer processes in Alexandrite. While further detailed investigation is necessary, based on our power efficiency measurements, a likely cause of the abrupt decrease in the laser efficiency with output coupling could be possibly attributed to ETU.
While monitoring the temporal characteristics of the laser output with a fast photodiode, we observed that, the Alexandrite laser worked in “pure cw” regime, when the cavity alignment was optimized to produce the maximum output power. Figure 6 shows a typical measurement of the laser output in “pure cw” regime, where we observe relatively stable cw laser output with a relative intensity noise of around 3%. On the other hand, when we slightly misaligned the cavity and/or change the separation between the curved mirrors (CM1-CM2 separation in Fig. 1), we could observe pulsed laser output. This phenomenon is known as self-Q-switching (SQS), and was first identified by I. Freund in 1968 [31]. During SQS operation, a laser produces pulsed output due to an inherent modulation mechanism inside the gain medium, without the need for any additional external modulation element inside the laser cavity.
Figure 7 shows the typical temporal output characteristics of the Alexandrite laser during the SQS operation regime taken using the 0.5% output coupler at the full pump power. For this specific case, the laser produced 7 μs long pulses at a repetition rate of 21 kHz. The laser average power was around 30 mW, corresponding to a pulse energy of 1.4 μJ and a peak power of 210 mW (0.15 duty cycle). Note that, the laser is quite stable in the SQS regime, meaning that the shot-to-shot variation in the pulse energy and pulsewidth is relatively low. At different SQS operation points, (using the 0.5% OC), we observed pulsewidth values ranging from 5 μs to 15 μs and pulse repetition rates in the 10-35 kHz range. These parameters depend on pump power and the cavity losses (output coupler percentage). Moreover, in the SQS mode, the laser transverse mode switched from being single-mode (TEM00) to a structured beam containing higher-order spatial modes. A typical mode profile of the laser output is shown in Fig. 7 (right). Unlike what has been reported in Cr:LiCAF [32], in SQS operation the optical spectrum width stayed similar to the cw case and had a FWHM of 3 nm near the central wavelength of (760 ± 10) nm.
Fig. 7 (Left and Middle) Measured temporal characteristics of the Alexandrite laser output in self-Q-switching (SQS) regime at different time scales. The pulsewidth and the SQS repetition rate was measured to be 7 microseconds and 21 kHz respectively. (Right): Measured sample output beam profile in the SQS regime. These are typical measurements and vary at different SQS operation points. An overexposed beam profile was chosen intentionally, to make the higher order modes more visible.
SQS was reported in many lasers including ruby [33], Nd:YAG [34], Cr:LiSAF [35] and Cr:LiCAF [32] lasers. SQS operation (also named as “spiking” [36], and “self-pulsation” [37]) has been reported in Alexandrite gain media earlier as well [36–38]. The physics behind SQS is not very well understood, and might be different for different gain media. For Cr3+-doped gain media such as Cr:LiCAF, Cr:LiSAF and ruby, the SQS effect is attributed to a nonlinear loss mechanism created by a time-dependent lens occurring inside the gain medium and originating from refractive index changes induced by the population inversion [35, 39–41]. In the case of Alexandrite, it is argued that the competition between the phonons and the laser field creates an unstable laser output [37, 42–44]. Moreover, the process has been asserted to depend on pump wavelength, pump intensity and cavity losses [37, 42–44]. From a practical perspective, SQS can be used to obtain μs long pulses with higher peak powers from the Alexandrite lasers, making these sources suitable for applications such as material processing, range finding, remote sensing, and nonlinear frequency conversion. On the other hand, as a drawback, SQS might create problems in obtaining long term stable cw or cw mode-locked operation in Alexandrite lasers. In our case, as a drawback of SQS, our pure cw operation was only stable for 5-10 minutes. After that, misalignments induced by thermal/mechanical instabilities have initiated SQS operation, and realignment of the cavity was required to restore the pure cw operation. SQS is not the main focus of this paper, and we note that further detailed studies are required to fully understand the underlying physics of SQS.
In order to estimate the intracavity loss level of the Alexandrite laser resonator, we performed both Caird [44] and Findlay-Clay [45, 47] analyses (Fig. 8). In the Caird analysis, the slope efficiency η of the laser can be estimated by using
where h is Planck’s constant, () is the laser (pump) photon frequency, is pumping efficiency, () is the emission (excited state absorption) cross section, T is the transmission of the output coupler, L is the total round trip loss, and η0 is the maximum (intrinsic) slope efficiency that can be obtained at high output coupling. Using Eq. (2), and the measured variation of the slope efficiency as a function of output coupling (Fig. 8 (a)), we determined the best-fit values of L and η0 to be (0.3 ± 0.1) % and (55 ± 10) %, respectively. Note that, in the Caird analysis, the slope efficiency values measured at high output coupling were excluded, due to their unexpected behavior, as we also discussed above. As an alternative method, we have also used the measured variation of the threshold pump power as a function of output coupler transmission (Fig. 8 (b)) to estimate the level of passive cavity losses. According to the Findlay-Clay analysis [45, 46], the laser threshold pump power (Pth) can be expressed as:where wp (wc) is the pump (cavity) beam waist, is the fluorescence lifetime of the upper laser level, and Ag is the ground state absorption of the Cr3+ ions. Other parameters in Eq. (3) were defined earlier. Self absorption losses due to the Cr3+ ion are insignificant around 750 nm, and hence, Ag can be neglected. Using Eq. (3), we have determined the best linear fit to the experimental data as (in mW units), and the round-trip intracavity losses as (0.2 ± 0.1) %. We see that the calculated values of passive losses using two different methods are in reasonable agreement with each other.Fig. 8 Left: Measured variation of the inverse of the slope efficiency as a function of the inverse of the output coupling (Caird analysis). Right: Variation of measured lasing threshold as a function of output coupler transmission (Findlay-Clay analysis).
We have also investigated the effect of temperature on the cw laser performance. Figure 9 shows the measured variation of the output power as a function of the crystal temperature for temperatures from 25 °C and 200 °C. First of all, as the laser crystal temperature increases, a shift in the free running laser wavelength of the Alexandrite laser was observed toward longer wavelengths. In particular, the free running laser wavelength shifted from around 760 nm to 805 nm, when the crystal temperature increased from 25 °C and 200 °C. This shift is quite expected and is due to a shift of the peak emission cross section wavelength with temperature, as it was also observed in our fluorescence intensity measurements (Fig. 4). However, note that the measured increase in the laser wavelength was not continuous and occurred in discrete steps. We believe that, the observed effect is due to a temperature induced birefringence in the Alexandrite gain medium, which created a spectral filter with a bandwidth of about 10 nm. The temperature induced birefringence rotates the polarization of the intracavity laser light by a small amount and then we observe losses from the Brewster-Brewster cut Alexandrite crystal for the TE component of the intracavity laser beam. Hence, the temperature induced birefringence filter prevents observation of a continuous shift in the laser wavelength, and an abrupt jump occurred at every 30 °C-interval or so, when the shift in the gain is strong enough to create a jump to the next transmission maxima of the induced filter. As we can see from Fig. 9, as the temperature of the Alexandrite crystal was increased, the obtainable cw output laser powers also decreased dramatically. Note that similar to the changes observed in the laser output wavelength, the laser output power did not change smoothly but in early discrete steps. We believe that this was caused in a similar fashion by the temperature induced birefringence effect combined by the Brewster-Brewster orientation of the gain medium.
Fig. 9 Measured variation of the cw output power and output wavelength of the Alexandrite laser as a function of the crystal temperature. The data were taken with a 0.5% output coupler at a pump power of around 150 mW.
To investigate the observed decrease in the laser output power with temperature in more detail, we also measured the laser efficiency curves at different crystal temperatures. Figure 10 shows the power efficiency curves taken at the crystal temperatures of 25 °C, 100 °C, and 200 °C, by using the 0.5% output coupler. First of all, note that, as the crystal temperature was increased, the lasing threshold values increased dramatically. For example, the lasing threshold increased 3.4 fold, from 28 mW to 95 mW as the Alexandrite crystal temperature increased from 25 °C to 200 °C. To understand the observed variation of the lasing threshold as a function temperature, we investigated the temperature dependence of the key parameters that influence the threshold pump power (, , and in Eq. (3). Among these, our measurements indicated that the fluorescent lifetime was the most sensitive to temperature variations. When the crystal temperature was increased from 25 °C to 200 °C, the fluorescence lifetime of the upper laser level ()decreased from 265 μs to 80 μs (a factor of 3.3 decrease). Hence, the observed 3.4-fold increase in the lasing threshold was predominantly due to the change in the fluorescence lifetime with temperature. This is clearly seen in Fig. 11 (left) which shows the temperature dependence of the threshold pump power and the inverse of the fluorescence lifetime, both normalized to their respective room-temperature values. In principle, both and are also temperature dependent. However, in this particular case, the temperature dependence of the cross sections was negligible, as can be seen, for example, in Fig. 11 (right) for the emission cross section near 750 nm, Moreover, the laser partly eliminates the observed decrease in the emission cross section with temperature by working at longer wavelengths at higher temperatures.
Fig. 10 Power efficiency curves measured at 25°C, 100 °C, and 200 °C using the 0.5% transmitting output coupler showed a monotonic decrease in the slope efficiency with increasing temperature.
Fig. 11 (Left) Measured variation of the threshold pump power and the inverse lifetime as a function of temperature, both normalized to their respective room-temperature values. (Right) Measured variation of the emission intensity at 750 nm as a function of temperature.
From Fig. 10, we also see a decrease in the measured laser slope efficiencies with increasing temperature. For example, the measured laser efficiency decreased from 36% to 12% (3 fold decrease), as the Alexandrite crystal temperature increased from 25 °C to 200 °C. The decrease in the slope efficiencies with temperature might be partly due to the increased role of excited state absorption at longer lasing wavelengths [26]. Moreover, there is a slight increase in the quantum defect due to the increase observed in the laser output wavelength from 760 nm to 805 nm. However, this effect will only decrease the efficiencies by about 6%. The observed decrease in the slope efficiencies (3 fold) should be attributed to the increased roll of additional loss mechanisms such as excited state absorption at longer wavelengths.
We have also investigated the cw tuning capability of the single-mode diode pumped Alexandrite laser. Figure 12 shows the variation of the laser output power with output wavelength measured by using the 0.5% output coupler, at an incident pump power of 170 mW. The data were taken at three different crystal temperatures to observe the effect of temperature increase on the wavelength tuning capability. The obtained output powers are slightly lower (for example, at the peak wavelength of 760 at 25 °C, the output power decreased from 48 mW to 34 mW), due the insertion of the birefringent tuning plate that introduced some unwanted losses. At room temperature, we could smoothly tune our cw laser wavelength from 736 nm to 795 nm. Note that, tuning on the short wavelength side was limited by the reflectivity bandwidth of the cavity high reflectors whose transmission increased below 750 nm (for example, the transmission was about 0.1% at 735 nm). Earlier studies reported tuning down to 701 nm in a pulsed high energy Alexandrite laser [10]. More studies are required with broader bandwidth high reflectors to investigate the cw tuning limit of Alexandrite on the short wavelength side. We also note here that increased role of excited state absorption and self absorption losses are also a limiting factor on the short wavelength end [48].
Fig. 12 Continuous-wave tuning curves of the alexandrite laser taken at the crystal temperatures of 25°C, 100°C, and 200°C by using the 0.5% output coupler. The pump power was 170 mW.
On the long wavelength side, the tuning limit was 795 nm at room temperature. A cw tuning upper limit of 816 nm was reported earlier from a cw Alexandrite laser operated at room temperature [22]. We believe that the limited pump power available in our study could be one of the factors in the observed decrease in the overall tuning range. On the other hand, we have seen that, by increasing the Alexandrite crystal temperature, the long wavelength tuning limit can be shifted up to 823 nm. Earlier, a tuning range extending up to 858 nm was reported by Kuper et al. from a pulsed Alexandrite laser at a crystal temperature of 513 °C [11].
5. Conclusions and summary
In this study, we have investigated in detail the continuous-wave and self-Q-switched operation of a low-threshold and efficient Alexandrite laser system. The Alexandrite laser was pumped by one single-mode diode providing up to 170 mW of pump power at 635 nm. Laser output powers as high as 48 mW, laser slope efficiencies of 36%, lasing thresholds as low as 13 mW were demonstrated during cw operation. Cavity round trip losses were estimated to be around 0.25% using the Caird and Findlay-Clay analyses. The cw laser output wavelength could be tuned from 736 nm to 795 nm at room temperature, and from 776 nm to 823 nm at 200 °C. We have also observed that, probably due to its long upper state lifetime, the Alexandrite laser had a high tendency for SQS operation. SQS could be observed by slight misalignment of the cavity or by slight change of the cavity curved mirror separation. During SQS, stable pulses with pulsewidths in the 5-15 μs range and pulse repetition rates in the 10-35 kHz range were generated. We believe that this compact, low-cost and efficient Alexandrite laser has the potential to become an attractive source of 800-nm radiation for several applications.
Acknowledgments
We thank Ilyes Baali for experimental help and Alfred Leitenstorfer from the University of Konstanz for providing some of the equipment that was used in this study. We acknowledge partial support from TÜBİTAK (The Scientific and Technological Research Council of Turkey: 113F199), European Union Marie Curie Career Integration Grant (PCIG11-GA-2012-321787), Alexander von Humboldt-Foundation, COST Action BM1205, and Center for Applied Photonics at Konstanz University.
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