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We present an investigation of a versatile pulsed laser source using a low power, gain-switched diode laser with independently variable repetition rate and pulse duration to seed an ultra-high gain Nd:YVO4 bounce geometry amplifier system at 1064nm. Small-signal gain as high as 50dB was demonstrated in a bounce geometry pre-amplifier from just 24W pumping, with good preservation of TEM00 beam quality. The single amplifier is shown to be limited by amplified spontaneous emission. Study is made of further scaling with a second power amplifier, achieving average output power of ~14W for a pulsed diode seed input of 188μW. This investigation provides some guidelines for using the bounce amplifier to obtain flexible pulse amplification of low-power seed sources to reach scientifically and commercially useful power levels.
© 2015 Optical Society of America
1. Introduction
Pulsed laser sources are useful for industrial applications such as drilling, cutting, high precision micromachining and laser marking [1–3]. Among laser sources, diode-pumped solid-state lasers such as Nd:YAG and Nd:YVO4 are proven sources of pulsed power with good efficiency and high beam quality. Most variable pulse rate solid-state lasers are achieved using active Q-switching, producing pulse durations typically ~10–100ns, although sub-nanosecond pulses can be achieved from passively Q-switched microchip Nd:YVO4, Nd:GdVO4 and Nd:YAG lasers with ultra-compact cavities [4–8]. However, Q-switched laser systems are limited in pulse versatility since the pulse rate, pulse energy and pulse duration are in general not independent but all interlinked by the stored inversion and cavity dynamics. Another approach to producing pulsed laser sources is by amplifying a seed source with variable pulse parameters.
In this work we consider the case where the seed is a gain-switched semiconductor laser diode. The gain-switching of laser diodes is controlled by the temporal form of the current applied to the laser, allowing flexibility and independence in pulse duration and pulse repetition rate. However, the peak power of gain-switched laser diodes is limited by optical damage, and the low duty cycle of the diode when operating with short temporal pulses can lead to very low average output power (typically sub-mW) [9,10]. To provide useful pulse energy and sufficient average output power for subsequent applications, an amplifier, or set of amplifiers, must be employed in a master oscillator power amplifier (MOPA) configuration. The amplifier system must provide extremely high amplification to produce a commercially useful output power e.g. for industrial material processing. The MOPA approach with diode seed is extensively used in high gain fibre laser systems, often using a series of fibre amplification stages [11,12]. A difficulty arises in fibre amplifiers due to high amplified spontaneous emission (ASE) and, more significantly, the onset of optical damage and fibre nonlinearity that typically limits the pulse energy and peak power achievable [13].
An alternative approach involves amplification using a bulk solid-state gain medium. However, barring recent work by Delen et al. [14], this approach usually provides limited gain. An exception to this is the ultra-high gain that can be achieved in the bounce amplifier geometry [15–22]. The bounce amplifier is a diode-side-pumped configuration, where the laser mode experiences total internal reflection (TIR) and amplification at a grazing incidence to the pump face of the crystal. By using a highly absorbing gain material with a correspondingly small absorption depth, such as Nd:YVO4 or Nd:GdVO4, the laser mode experiences a region of high inversion at the pump face, allowing efficient power extraction and extraordinarily high gain levels. High average powers of over 100W have been achieved using Nd:GdVO4 in a MOPA bounce configuration [16]. Nd:YVO4 in the bounce geometry has achieved high output powers with good beam quality in both CW and in different pulsed regimes [17–22].
In this work, we present a study of the flexibility of using the bounce geometry as an ultra-high gain (50dB) amplifier for creation of a versatile variable repetition rate and pulse duration system. The seed source in this study is a gain-switched semiconductor diode laser at 1064nm. Prior work has been performed on the bounce amplifier MOPA configuration using sub-ns pulses from a passively Q-switched Nd:YAG laser [20] and using a passively modelocked (picosecond) Nd:YVO4 [21] as the seed sources. In this paper, we provide a more systematic investigation of the gain and power scaling of the bounce amplifier when using a gain-switched diode seed operated at different (ns-class) pulse durations, and repetition rates up to 2MHz. We study the effects of varying the bounce angle and pump parameters using a pre-amplifier based on diode-pumped Nd:YVO4 slab for providing ultra-high gain when using μW seed powers. We find the limits of the amplifier are due to ASE, so a second power amplifier is utilised to further boost the average power levels. With 188μW seed power we obtain an output of ~14W. The flexible pulse duration and pulse repetition rates in this system allow for the possibility of optimising this type of diode-seeded bulk laser amplifier system for a wide range of processes requiring flexible pulse and high peak power format.
2. Experimental system
The schematic of the investigated flexible MOPA pulse source is shown in Fig. 1. The system comprised of a pulsed laser diode seed at 1064nm, a Nd:YVO4 pre-amplifier and a Nd:YVO4 power amplifier, both in the bounce geometry.
Fig. 1 Schematic diagram of the MOPA system with gain-switched diode laser seeding a pre-amplifier and power amplifier, both in the bounce geometry. HCL and VCL are horizontal and vertical cylindrical lenses, respectively; OI are optical isolators and HWP are half waveplates.
2.1 Pulsed laser diode seed system
The pulsed seed source used in this system was a semiconductor laser diode, connected to a pulsed electrical driving circuit for gain-switched operation. The electrical drive circuit was capable of generating pulse durations from 3.5ns to continuous wave (CW) and with adjustable pulse repetition rate, ranging from single shot up to 2MHz. The minimum pulse duration of this system was limited by the finite inductance of the laser diode and our cabling system, rather than being a fundamental limit.
The laser diode was spatially single mode and had a nominal free-running wavelength of 1064nm, and CW rated power of 200mW. It was housed in a 9mm TO-can package and temperature stabilised with a thermo-electric (Peltier) device. It was collimated with an aspheric lens producing an elliptical (~3:1 ratio) spatial output but with near-diffraction-limited beam quality. The diode laser was formed of a Fabry-Perot cavity and had a spectrally multimode output. Whilst the central wavelength of the spectral band could be temperature tuned, mode hopping and discrete longitudinal mode spectrum meant that the wavelength could not be continuously tuned, nor fully matched in general to the narrow (<1nm) gain bandwidth of the Nd:YVO4 amplifiers. The wavelength of the laser diode was therefore tuned using an external diffraction grating, the diode operating with an external cavity in Littman-Metcalf configuration [23]. The spectrum produced was single longitudinal mode and its spectral bandwidth measured to be <1GHz (the limit of spectral resolution of our measurement etalon). The seed laser was wavelength tuned to match the peak in the spectrum of the ASE from the pre-amplifier gain module, which occurred at a centre wavelength of 1064.2 nm (see Fig. 2).
Fig. 2 Spectrum of the wavelength-tuned diode seed laser and its matching to the ASE of the pre-amplifier gain medium.
The power of the seed laser could be varied by the electrical driving circuit, but fuller variation of power (by several orders of magnitude) in our studies was accomplished by using an external optical attenuator based on adjusting a retardation half-waveplate (HWP) placed before the first optical isolator (OI) (see Fig. 1). The OI was included to minimise unwanted feedback to the diode seed from the ultra-high gain amplifier that could affect the gain measurements and potentially cause optical damage to the diode laser itself. A second OI placed between the pre- and power amplifiers provided additional feedback protection for this study.
2.2 Nd:YVO4 bounce pre-amplifier and power amplifier
The laser crystals used in both pre-amplifier and power amplifier in this study were 1.1 at. % Nd:YVO4 slabs with dimensions of 20 x 5 x 2mm. Each crystal was side-pumped by a laser diode bar emitting at 808nm, on one of the 20 x 2mm faces, which was anti-reflection (AR) coated for the pump wavelength. The pump radiation was polarised parallel to the high-absorbing c-axis of the Nd:YVO4 crystal. Heat was extracted from the crystal by conduction cooling via the large top and bottom 20 x 5mm faces. The two end laser faces (5 x 2mm) were AR coated for the laser wavelength (1064nm) and angled to minimise parasitic lasing effects from unwanted internal oscillations in the slab crystal.
For the pre-amplifier, the pump diode bar was focused onto the crystal by a vertical cylindrical lens (VCLD1) with f = 6.35mm, creating a pump distribution at the crystal face with length l ~15mm and height h ~50μm. A pair of horizontal cylindrical lenses (HCL1, HCL2) with f = 200mm, −50mm formed a reducing telescope to create a horizontal seed mode diameter ~1mm at the pre-amplifier and a vertical cylindrical lens (VCL1) with f = 50mm was used to spatially match the seed beam to the vertically narrow gain region of the pre-amplifier. A second vertical lens VCL2 (f = 50mm) placed after the pre-amplifier was used to re-collimate the beam in the vertical dimension.
For the power amplifier, the pump diode bar was focused onto the crystal by a vertical cylindrical lens (VCLD2) with f = 25mm producing pump height h ~200μm. The power amplifier was placed at a distance approximately 50cm away from the preamplifier. This distance was found to be useful in order to reduce the acceptance angle of ASE from the pre-amplifier system entering the power amplifier. A second HWP combined with a second OI was used to control the power to the power amplifier and was also used to minimise feedback from the power amplifier to the pre-amplifier. The second OI introduced transmission losses of approximately 10% to the system. A pair of positive and negative horizontal cylindrical lenses (HCL3, HCL4) with f = 100, −50mm were used as a telescope to adjust the size of the beam horizontally by a factor of two and a vertical cylindrical lens (VCL3) of focal length 100mm was used to adjust the vertical size of the beam into the power amplifier.
3. Theoretical analysis of the bounce amplifier
The pre-amplifier and the power amplifier both utilised the bounce geometry, as shown in Fig. 1. A key feature of the bounce geometry involves the laser mode making a path of TIR, or bounce, with a small angle (θ) at the pump face of the gain medium. A more detailed schematic of the bounce amplifier geometry is depicted in Fig. 3, showing the central path of the beam to be amplified travelling through the crystal (including refraction at the crystal end faces). The crystal is diode pumped over a length l of the pump face, and the crystal absorption depth (x0) for the pump indicated by the shaded region. The pump intensity is maximum at the pump face of the crystal (x = 0) and decreases exponentially with penetration distance (x). The local gain coefficient of the amplifier is given by: α(x) = α0exp(-x/x0), where α0 is the gain coefficient at the pump face, and the absorption depth of pump beam x0 = 1/αp where αp is the pump absorption coefficient.
Fig. 3 Bounce amplifier geometry showing central beam path with bounce angle θ, and pumped length of crystal l, with absorption depth x0.
The small signal gain G = exp(g) of the bounce amplification can be calculated by integrating along the beam path to find the integrated gain-length coefficient: g = ∫α(r)dr where α(r) is the local gain coefficient. The gain-length product coefficient for the central path of the beam can be integrated to obtain:
The diode pump is assumed constant over the length of the irradiated pump face l, where l is typically less than the length of the crystal to avoid diffraction effects at the edges of the slab. Equation (1) shows the gain coefficient g(θ) depends strongly on the bounce angle θ, especially at shallow incidence angles, and since the gain G is exponentially related to g, the gain can be strongly dependent on angle. The calculation for Eq. (1) is for the central ray which experiences the maximum gain through the crystal. The actual gain experienced by the laser mode will be the net amplification of the whole 2D transverse laser mode distribution; hence Eq. (1) will be an upper limit on the gain.Another spatial consideration of the bounce geometry is the projection of the laser mode on the pump face. For a bounce angle θ, a laser beam of transverse diameter d0 will have a projection length across the pump face ld = d0 / sin (θ) (~d0 /θ at small angles). For optimal extraction the laser mode projection length, ld should be matched to the length of the pump beam l which can be controlled by input mode size d0 and / or bounce angle θ. However, care must be taken that the laser mode does not excessively overspill the pump length or, more critically, the slab face itself, causing diffractive clipping that will degrade beam quality. A further spatial issue is that for spatial averaging of the bounce to apply across the beam width, the bounce angle should sufficiently exceed the angle defined by the angular extent of the pump region (θp = x0/l). For our experimental case, the absorption coefficient of Nd:YVO4 is αp ~30cm−1 (x0 ~0.33mm) and the pump length l ~15mm, giving θp = x0 / l, ~22 mrad or 1.26°. The minimum bounce angle tested in our experiments was therefore set as θ = 3°.
4. Experimental results
Investigation was made of the pre-amplifier across a range of bounce angles, pump powers, and seed inputs. Subsequent study was made of the addition of the power amplifier for increasing the output power of the system.
4.1 Results of bounce pre-amplifier
The gain of the pre-amplifier at different bounce angles was investigated initially with a 1mW CW seed power. Figure 4 shows the variation of output power against pump power for three different bounce angles (θ) at 3°, 5° and 7°. The output power at the smaller angle was higher than at the larger angles. This is in agreement with Eq. (1) and can be expected since at small angles the laser mode experiences a longer path at the high inversion region near the pump face. The amplified power measured above ~15W diode pumping exceeds 103, but this gain is already saturated and very far from the small-signal value of the amplifier. It is also observed that at the smallest bounce angle (θ = 3°) the laser spatial mode displays some weak clipping in the horizontal direction due to overspill from the pump region and at the crystal edges. The projection length onto the pump face of the seed beam at θ = 3° with input seed horizontal diameter d0 ~1mm is ld = d0 / sin (θ) ~19.1mm, which is close to the slab crystal length of 20mm.
To preserve spatial quality and to minimise any diffractive clipping effect, the experiment to assess the small signal gain of the amplifier was set up with the θ = 5° bounce angle. The projection length onto the pump face of a 1mm diameter beam at this angle is ld ~11.5mm, reasonably well-matched to the pump width (~15mm) and safely within the slab length of 20mm.
Figure 5 shows the gain of the amplifier with varying diode seed average power (using the HWP and OI) for fixed seed pulse format of 500ns pulse duration and at a repetition rate of 100kHz. At a seed average power of 2μW, a small signal gain of ~50dB was measured at 24W pumping and small-signal gain ~39 dB was measured at 12W pump power.
Fig. 5 Gain of the amplifier in dB as a function of seed power at 5° bounce angle, for two different pump powers.
The M2 beam quality of the amplified beam was determined using the ISO 11146-1 method, based on the second moment beam size. The input seed laser was measured to be Mx2 = 1.1 and My2 = 1.3 in a horizontal and vertical, respectively. The measured beam quality after passing through the pre-amplifier at 24W pump power was Mx2 = 1.1 and My2 = 1.4. This result shows that the beam quality is well preserved in the pre-amplifier.
The achievement of such a high small-signal gain of ~50dB (105) is remarkable for bulk amplifier technology and shows that micro-Watt seed powers can be amplified to near-Watt level powers, with preservation of high spatial quality. To quantify further the power scaling potential of a single high gain amplifier stage, the seed power was varied by adjusting the seed pulse duration (10ns; 100ns; 1000ns; 5000ns) at a fixed pulse rate of 100kHz. The corresponding input seed powers to the pre-amplifier were 21μW; 188μW; 1.62mW; and 8.1mW, for the four pulse duration cases, respectively.
Figure 6 shows the average output power against pump power at the 5° bounce angle for these four seed input cases. Output power of 6.5W is achieved at 32W pump power for the highest input seed power (8.1mW). However, roll-over at high pump powers is observed for all cases. The power roll-over can be attributed to ASE that becomes comparatively high as the pump power increases, extracting significant power and competing for gain with the seed beam. As the seed pulse duration and hence input seed power increases, the output power rollover is observed to occur at higher pumping power. This is explained by increased gain saturation that occurs for large seed input that reduces the level of ASE, compared to the lower seed case. In general, however, the single amplifier becomes ASE limited and for higher power scaling it is therefore necessary to incorporate a further power amplifier module.
Fig. 6 Output power versus pump power for different input seed pulse durations (and hence seed powers).
4.2. Results of bounce power amplifier
The details of the power amplifier are given in Section 2.2. The main difference compared to the pre-amplifier is the longer diode vertical focusing lens (VCLD2) used (f = 25mm, compared to f = 6.35mm for pre-amplifier) to give a larger vertical pump size, as this amplifier is more concerned with power extraction than achieving the highest gain. With less intensive pumping the ASE issues are also reduced. The power amplifier was operated at 5° bounce angle, as with the pre-amplifier. The pre-amplifier pump power was fixed at 24W in the following experiments.
Figure 7 shows the average output power after the power amplifier stage as a function of diode pump powers for different seed pulse durations (FWHM). The variation of seed pulse duration controls proportionately the average power of the seed. A maximum output power of ~14W was obtained from 100ns (188μW) seed pulse and 55W pump power into the power amplifier.
The average output power after the power amplifier with varying repetition rate is shown in Fig. 8(a), for two different seed pulse durations (3.5ns and 10ns), at 25W and 50W pumping of the power amplifier. Figure 8(b) displays the corresponding peak power for the case of the 3.5ns pulse seed. Figure 8(a) shows that the average output power increases with repetition rate, since the seed input power is increasing.
Fig. 8 (a) Average output power with varying repetition rate. (b) Peak pulse power with varying repetition rate for fixed seed input duration 3.5ns. Note that the input seed average power changes proportionately with repetition rate change.
The output power increases rapidly at low repetition rates (quasi-linearly) but tends to reach an asymptotic limit at high repetition rates due to gain saturation. The peak power is highest at lower repetition rate, as shown in Fig. 8(b), since the time between pulses was longer, accessing a higher population inversion at the time the pulse arrives at the gain medium. A peak power of 42kW was obtained at a repetition rate of 50kHz at 50W pumping to power amplifier.
At 25 W pumping to the power amplifier, the beam quality was measured to be Mx2 = 1.3 and M y 2 = 1.1 and at 50 W pumping, the beam quality M x 2 = 1.5 and M y 2 = 1.1. Further optimisation of the system would improve the beam quality. No obvious parasitic lasing was observed. It is noted that at full pump power (55W), the thermal lensing due to the power amplifier in the horizontal direction was very strong, creating a thermally-induced focal length of approximately 60 mm. The output beam however was readily re-collimated using a suitable HCL at an appropriate distance from the amplifier.
Figure 9 shows temporal profiles comparing the laser diode seed input and amplified output pulses after two amplification stages. Figure 9(a) shows the “Gaussian-like” temporal profile for a seed with pulse duration of 3.5ns at 100kHz repetition rate and shows that the changes in pulse shape and pulse duration were small after amplification. Figure 9(b) shows the temporal profiles for “square-shaped” seed pulse duration of 60ns and shows the reshaping of the pulse due to the effect of gain saturation where the trailing pulse edge sees lower gain than the leading edge. This effect was even more apparent at low repetition rates, where the gain is higher and the gain saturation changes more appreciably during the pulse duration.
Fig. 9 Temporal profile of laser diode seed and amplified pulse. (a) Seed pulse duration of 3.5ns, showing negligible pulse shortening. (b) Seed pulse duration of 60ns, showing the FWHM pulse duration decrease to 21 ns due to pulse reshaping by gain saturation.
5. Conclusion
We have presented a study of using a low power, gain-switched diode seed and ultra-high gain bounce geometry Nd:YVO4 amplifier system to create a versatile pulsed source at 1064nm. The pulse duration and repetition rate could be independently varied by adjusting the seed laser, limited only by the diode driving circuit. The output power and gain obtained from different bounce angles and pump powers from a pre-amplifier was investigated experimentally. Small signal gain of 50dB was demonstrated from a pre-amplifier with just 24W pumping. At higher pumping powers, the amplifier becomes limited by competition from ASE. To further scale the power of a sub-mW seed it was necessary to use a second bounce geometry power amplifier. With power amplifier, the average output power of ~14W was achieved for a diode seed input with 188μW power, 100ns pulse duration at 100kHz. The flexibility of the peak power, pulse duration and repetition rate in such a simple amplifier system can be useful for various applications, including precision micromachining. Higher peak powers can be obtained by using even shorter duration seed pulses (sub-ns and ps) than used in this study. This study provides some useful guidelines for the general use of the bounce geometry amplifier for flexible pulse amplification of low power seed sources to scientifically and commercially useful power levels.
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