We have developed a high-gain, high-peak-power laser amplifier at an eye-safe 1.55 μm wavelength using an Er,Yb:glass planar waveguide for wind sensing coherent Doppler lidars (CDLs). Our planar waveguide is free from stimulated Brillouin scattering and realizes high gain thanks to its multi-bounce optical-path configuration. A peak power of 5.5 kW with a pulse energy of 3.2 mJ is achieved at the repetition frequency of 4 kHz, which leads to an average power of 12.8 W. The gain is more than 23 dB. The wind sensing at more than 30 km is demonstrated with a CDL using the developed amplifier.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
A coherent Doppler lidar (CDL) [1,2] provides real-time remote sensing of wind velocity even in clear air conditions, and is a promising technology for meteorological and aviation applications. Recently, the need for wind sensing CDLs has been increasing, especially for wind monitoring around airports for safer landings , the detection of clear air turbulence for flight safety at cruising altitude , and wide-area wind mapping for efficient wind energy production . For these applications, long-range wind measurement with high data availability is required. To meet this requirement, a high-peak-power, high-average-power (i.e., high-repetition-frequency) output is desired for the pulsed laser transmitter, in addition to the requirement for stable single-frequency operation (laser linewidth < 1 MHz), high beam quality (M2 < 1.5), and relatively long pulse duration (pulse width > 0.1 μs).
From the viewpoint of eye safety, the 1.55 μm wavelength region is attractive, and Er-doped fiber amplifiers with single-mode optical fibers have been widely used for the transmitters of wind sensing CDLs because of their high efficiency, very good output beam quality, ease of use, and so on [6–11]. However, it is difficult for a single-mode fiber amplifier to provide a high-peak-power pulse at a single frequency because of nonlinear effects, especially stimulated Brillouin scattering (SBS). Improved fiber amplifiers have been studied to overcome the peak-power limitation due to SBS, for example, the coherent beam combining amplifier [12,13], rare-earth ion highly-doped Er,Yb doped fibers [14,15], large mode area fibers [16–23], multi-core fibers , and photonic-crystal fibers . However, 1 kW or higher output peak power with more than 1 mJ pulse energy and near-diffraction-limit beam quality is still challenging for fiber amplifiers, especially for the narrow bandwidth lasers which are suited to CDLs.
A solid-state laser can overcome this issue, and laser transmitters for CDLs based on this type of laser have been developed for wavelengths of 1.06 μm [26,27], 2 μm [28–41], and 1.6 μm [42,43]. For a wavelength of 1.55 μm, Er,Yb:glass is the most popular laser medium. This medium has been studied for a long time [44,45], and a high-peak-power transmitter for a CDL using an Er,Yb:glass rod has been reported [46,47]. However, the relatively small emission cross section, low thermal conductivity, and low thermal fracture limit of the Er,Yb:glass have prevented this from achieving a high-average-power output.
From the above mentioned background, we have developed a laser amplifier using an Er,Yb:glass planar waveguide to provide a higher peak-power output than the conventional fiber amplifiers can emit. Planar waveguides have many attractive features which neither fiber amplifiers nor Er,Yb:glass rod lasers have. Our planar waveguide structure is good at dissipating heat because the waveguide is in contact with a heatsink over a large area, and the distance between the waveguide core and the heatsink is very small. In addition, the large cooling surface of the planar waveguide structure permits high-average-power, high-density pumping, and consequently permits high-average-power output. This means the planar waveguide structure can avoid the demerit of Er,Yb:glass of having a low thermal fracture limit, which rod lasers cannot [45,46]. The nonlinear effects are also reduced in the planar waveguide because of its large beam area, which is a limiting factor for fiber amplifiers, as mentioned above. Despite the propagating beam having a large beam area in the planar waveguide, good beam quality is maintained because the difference between the refractive indices of the core and the cladding layers is adjusted so as to allow only a small number of modes to propagate in the planar waveguide.
Our planar waveguide also uses a multi-bounce optical path configuration, which yields high gain despite the small stimulated emission cross section of the Er,Yb:glass. The stimulated emission cross section of the Er,Yb:glass is on the order of 10−21 cm2, which is two orders of magnitude less than that for Nd:YAG. Therefore, Er,Yb:glass needs a long propagation path to realize a high gain amplifier, and the multi-bounce optical path configuration can achieve this more easily than a rod type configuration. Thanks to these features of the planar waveguide configuration, optical pulse energies greater than 1 mJ are obtained, which are suitable for a long range coherent Doppler lidar.
In this paper, details of the Er,Yb:glass planar waveguide and its demonstrated laser amplification are presented. In the past, there have been reports on planar waveguide amplifiers for integrated path differential absorption lidars for the 1.6 μm wavelength [48,49]. However, the amplifier presented in this paper is the first one which can be applied to a wind sensing CDL. A demonstration of long range wind sensing using this amplifier is also presented. Some of the content of this paper has previously appeared in several conference papers [50–53], but we here present new information essential to achieving the high performance required of the laser transmitter of a wind sensing CDL, including (i) the detailed verification of the beam quality, which is extremely important for a CDL, (ii) how the output power was improved over that reported in the earlier papers, (iii) the pulse shaping technique, and (iv) the detail on how to suppress the amplified spontaneous emission (ASE). Further, new wind sensing results which have not previously been presented are also introduced.
2. Er,Yb:glass planar waveguide amplifier
2.1 Waveguide structure
Figure 1 shows a cross sectional view of the Er,Yb:glass planar waveguide. The double-clad waveguide consists of an Er,Yb:phosphate glass core, a non-doped phosphate glass inner cladding (1st upper cladding), and optical glass outer cladding layers (under cladding and 2nd upper cladding). The waveguide is attached to a water-cooled copper heat sink using an adhesive. The concentrations of Er and Yb ions are 0.87wt% of Er2O3 and 16.3wt% of Yb2O3, respectively. The size of the waveguide is 23 mm x 30 mm x 8 mm. The thicknesses of the core and 1st upper cladding are 19 μm and 105 μm, respectively. The signal beam propagates in the core, and the pumping beam is confined to the 1st upper cladding and the core. To obtain low-order guided-mode propagation of the signal beam, the refractive index of the core is only slightly larger than that of the 1st upper cladding, by 0.5%. The numerical aperture (NA) of the signal beam is approximately 0.1, and that of the pumping beam is approximately 0.45.
2.2 Amplification system
2.2.1 Configuration and laser amplification
Figure 2 shows the configuration of the laser amplifier. The two side surfaces of the waveguide are polished and coated with an anti-reflection (AR) coating at 940 nm for the pump light. A total of 28 pig-tail 940 nm laser diodes (LDs) are used for pumping, and the two fiber arrays contain 14 optical fibers to connect to the respective laser diodes. The optical fibers have an NA of 0.22 and a core diameter of 105 μm. The fiber arrays are placed very close to the planar waveguide to launch the pumping beams into it. High coupling efficiency is expected because both the NA and the core diameter of the optical fibers are sufficiently smaller than those of the planar waveguide. The total maximum pumping power is 312 W. The pumping beams propagate within the waveguide on a single pass, spread in the planar direction due to diffraction corresponding to the NA of the input pumping beams, and are homogenized around the center part of the waveguide.
The incident signal pulses are focused on an AR area of the side face by a cylindrical lens as shown, and thence coupled into the waveguide. The signal pulses propagate in the waveguide core in a guided-mode in the waveguide direction and by free-space propagation in the planar direction. The incident signal beam propagates back and forth between two high-reflectivity (HR) coatings on both faces, which are slightly off-parallel, as shown in Fig. 2. The amplified signal beam exits from the AR area whence it entered, and is separated from the incident signal beam by an optical circulator which comprises polarization beam splitters (PBS) and a Faraday rotator. Because the planar waveguide amplifier has a very high small-signal gain of more than 30 dB, parasitic oscillation can occur due to slight reflections all around the waveguide. To suppress parasitic oscillation in the waveguide, all four faces of the waveguide are set off-parallel. Also, the optical components, including the AR coated devices, are placed out of perpendicular to the optical axis.
The beam size of the signal propagating in the waveguide is larger than that in a conventional single-mode fiber by more than 200 times in the planar direction and by twice in the waveguide thickness direction. A 90 W SBS threshold of the peak power for a single-mode fiber has been demonstrated with a pulse width of 600 ns . Based simply on scaling the beam area, this leads to an SBS threshold for the planar waveguide of more than 10 kW with the same pulse width.
2.2.2 Input signal
The input signal beam to the laser amplifier is supplied by the following devices: a 1550 nm distributed-feed-back (DFB) fiber laser (Koheras Basik E15, NKT Photonics) used as the narrow-linewidth (<100 Hz) signal source, and an acousto-optic modulator (AOM) to chop it into pulses with a repetition frequency of 4 kHz. The wavelength of 1550 nm was selected in consideration of the gain and absorption in the overall laser amplification system (including the fiber amplifier and the planar waveguide amplifier). The signal pulses are amplified by an optical fiber amplifier with a gain of more than 40 dB. The signal beam has an average power of 60 mW (15 μJ/pulse) after the fiber amplifier , and is used as the input signal to the planar waveguide amplifier.
2.2.3 Pulse shaping
Pulse shaping is necessary since the optical pulse-shape is distorted during amplification because of gain saturation in the fiber amplifier and the planar waveguide amplifier [54,55]. In other words, the leading edge of the pulse depletes the population inversion, and thus experiences a higher gain than the trailing edge. Such pulse distortion leads to a shortened pulse duration and a higher peak power that results in stimulated Brillouin scattering.
In our amplification system, the pulse-shape distortion mainly affects the available output power from the fiber amplifier, and therefore the input pulse-shape of the fiber amplifier needs to be attended to. Here, the AOM is driven by the ramp-like waveform shown in Fig. 3 to compensate the pulse-shape distortion. The pulse shape of the output from the fiber amplifier has been presented in , and is similar to the output pulse shape from the planar waveguide amplifier as can be seen later in Section 3 of this paper.
The entire (bottom-to-bottom) duration of 2 μs corresponds to the distance light travels in a round trip of 300 m, which is the requirement for our CDL’s range resolution (see Section 4). The pulse duration is the key for the range resolution of a lidar, and a fiber amplifier with a shorter pulse width of about 1 ns, corresponding to a higher range resolution of 15 cm, has been demonstrated . For the wind sensing CDL, a shorter pulse means that the spectrum of the received heterodyne-detected signal is broadened, and this reduces the peak intensity in the spectrum, and consequently degrades the wind sensing performance (maximum measurable range, etc.). Therefore, a longer pulse is somewhat better in terms of increasing the signal to noise ratio. On the other hand, when the pulse becomes too long, the standard deviation of the wind velocity within the probed atmospheric volume becomes large because of the effects of turbulence . This broadens the spectrum of the received heterodyne-detected signal. Therefore, there is a trade-off between the range resolution and the maximum measurable range. The above mentioned pulse duration of 2 μs has been set in consideration of this trade-off.
2.2.4 ASE suppression
When amplifying a 1550 nm signal beam with high gain, the ASE at around 1535 nm can be a problem. High intensity pumping of Er:phosphate, which has relatively small cooperative-upconversion coefficients , causes a large population inversion and generates a large gain at 1535 nm, more than at 1550 nm. To suppress ASE around the wavelength of 1535 nm, we introduce loss to the ASE via the 1550 nm HR coatings. The reflectance at 1535 nm is held to less than 66% while maintaining high reflectance at 1550 nm due to the difference in single-path gain between 1550 nm and 1535 nm. The small signal gain g0L is calculated by , where L is propagation length (23 mm), is stimulated emission cross section, is absorption cross section, is erbium ion density in the upper laser manifold, and is that in the lower laser manifold. The values of and are estimated from Ref . such that at 1535 nm cm2 and cm2, while at 1550 nm cm2 and cm2, respectively. Assuming a maximum pumping power of 300 W, 82% of the erbium ions are excited to the upper laser manifold. From these assumptions, the single-path gain is calculated as 2.55 at 1535 nm and 1.69 at 1550 nm. Therefore, the gain at 1550 nm is 66% of that at 1535 nm. This is offset by the losses at 1535 nm at the multiple reflection points, leading to the overall gain at 1535 nm being lower than that at 1550 nm, thus suppressing the unwanted ASE.
3. Results for laser amplification
3.1 ASE output characteristic
To evaluate the waveguide’s performance, we measured the ASE output of the waveguide in the absence of a signal light. Figures 4(a) and 4(b) show the measured ASE output power and its spectrum, respectively. A maximum output power of 34 W at a pump power of 312 W is obtained. The wavelengths of the ASE are mainly 1548 nm and 1553 nm. ASE at 1535 nm is completely suppressed.
3.2 Signal amplification performance
Figure 5(a) shows the measured average amplified power. Because ASE is mixed with the amplified signal, we have extracted the signal using a narrow line-width band-pass filter at 1550 nm. The maximum average output power of the amplified signal is 12.8 W (3.2 mJ/pulse) at a pump power of 230 W, while there is 2.5 W of ASE power. The slope efficiency is 6.6%. This is lower than the optical conversion efficiency of 16.7% of an Er,Yb:fiber amplifier . The reason for this difference in efficiencies is that the pump power is applied to many locations within the planar waveguide which the signal beam does not pass through, in effect “wasting” some of the pump energy. In comparison, a slope efficiency of 20.3% is reported for rod-type lasers in full multimode operation, whereas their slope efficiency drops to approximately 2.5% if the cavity is reconfigured to obtain the TEM00 mode operation and long 200 ns pulse duration required for CDL applications . This planar waveguide therefore achieves higher efficiency than a rod type for CDL applications.
The highest signal output power of 12.8 W with 2.5 W of ASE power shown in Fig. 5(a) is an improvement over the values presented in the previous conference papers [50,51] (signal: 7.6 W, ASE: 6.5 W with the same pumping conditions), although the two amplifiers were produced using the same design procedure and the same manufacturing process. This is simply a consequence of the uneven quality of the HR coating used for ASE suppression.
The output spectrum at the highest output power (12.8 W) is shown in Fig. 5(b). The peak at 1550 nm corresponds to the signal, and the other peaks are ASE. Although small peaks are found at around 1545 nm, the ASE is effectively suppressed overall. The inset is an enlargement of the area around the 1550 nm peak. Because the fiber amplifiers used for the input signal to the waveguide amplifier have narrow-band filters to remove ASE, the residual ASE seen around the signal peak is also well suppressed. Therefore, the ASE component of the signal power in Fig. 5(a) is negligibly small.
The output pulse shape is shown in Fig. 5(c), and the measured pulse duration is 579 ns after amplification, which leads to approximately 5.5 kW peak power without any indication of SBS occurring. The gain is more than 23 dB.
The beam quality is an extremely important parameter for the laser transmitter of a CDL. To measure the beam quality of the output signal, we inserted a focusing lens (f = 200 mm) and measured the beam width using a charge coupled device (CCD) camera after the lens while moving the CCD camera around the focal point (at 205 mm from the lens). The results are shown in Fig. 6(a). X corresponds to the horizontal direction (parallel to the layers), and Y corresponds to the vertical direction (perpendicular to the layers). The beam quality factor M2 is calculated as 1.59 for the horizontal direction and 1.05 for the vertical direction. The geometric mean of vertical and horizontal M2 is 1.29 (). Figure 6(b) shows the beam pattern captured by the CCD camera around the focal point.
4. Wind sensing demonstration
4.1 Configuration of CDL
A wind sensing CDL using this planar waveguide amplifier has been demonstrated. The configuration of the CDL is shown in Fig. 7. The main units of the system are as follows: an optical transmitter/receiver unit , an optical antenna unit, 1480 nm LDs, a chiller, a power supply for the LDs and a signal processor unit. The optical antenna unit contains the planar waveguide amplifier acting as the main amplifier. The other components in the optical antenna unit are a 2nd stage amplifier, a circulator, a telescope, and pumping LDs at 940 nm. The optical transmitter/receiver unit provides the basic functions of the wind sensing CDL, and incorporates a 1550 nm DFB fiber laser, a balanced receiver, an AOM, and an Erbium doped fiber amplifier (EDFA) providing the 1st stage of amplification. The output of the DFB fiber laser is divided into a signal beam and a local oscillator beam for the heterodyne detector. The signal beam is pulse-modulated and frequency-shifted by the AOM. The optical pulses are amplified to a peak power of 5 W at the 1st stage amplifier.
The 2nd stage amplifier is an Er,Yb doped fiber (EYDF) pumped by 1.48 μm LDs, which is described in . This amplifier increases the peak power to 40 W. The main amplifier (the planar waveguide amplifier) is operated with a peak power of 2.4 kW, a pulse duration of 580 ns (i.e. a pulse energy of 1.4 mJ), and the pulse repetition frequency is 4 kHz. The main amplifier was produced to the same design and with the same manufacturing process as the amplifier presented in Section 3, but the reduction in peak power from the 5.5 kW in Section 3 to the above 2.4 kW was unavoidable because of the uneven quality of the HR coating used for ASE suppression (see also Section 3). The output of the main amplifier is transmitted to the atmosphere through a circulator and a telescope. The aperture of the telescope is 150 mm. The circulator consists of a PBS and a 1/4-wave plate.
The transmitted light is backscattered by aerosol particles in the atmosphere, and the backscattered light is passed via an optical fiber to the balanced receiver for heterodyne detection. The detected signals are processed in the signal processor unit. In the signal processing, the signals are gated by timed gates corresponding to a set of ranges, and a periodogram (almost the same as a spectrum) for each range is calculated by fast Fourier transformation. Incoherent accumulation is used to improve the detection probability and the velocity estimation accuracy. The accumulated periodograms are post-processed and the noise floor is rejected. The Line of Sight (LOS) velocity of each range is estimated from the first moment of the post-processed periodogram around the peak frequency. The peak frequency search range corresponds to the LOS velocity search range from –30 m/s to + 30 m/s.
The total system efficiency of the CDL system is about 10%, including the theoretical limit of 40% of the heterodyne detector , the quantum efficiency of 80% of the balanced receiver, the effect of the beam quality, the insertion loss of the optics, and so on.
4.2 Demonstration results
We evaluated the performance of the CDL for wind sensing. Some examples of this wind sensing were presented in , however in this paper we present new results which did not appear in that paper. The date of the wind sensing experiments reported in  is November 12th 2011. The dates of the experiments reported in this paper are November 16th 2011 (Figs. 8(a)–8(c)), and January 19th 2013 (Figs. 9(a) and 9(b)).
4.2.1 Horizontal path wind sensing
The range dependence of the detectability and line of sight (LOS) velocity for horizontal path measurement are shown in Figs. 8(a) and 8(b). The 100 consecutive data points collected are plotted in these figures. The wind sensing in this paper is performed without any scanning and in beam staring mode. The beam elevation angle is kept to a few degrees to keep the measurement region within the atmospheric boundary layer. The duration of the gate is 2 μs, which corresponds to a 300 m range resolution. The number of accumulated pulses is 16,000 for each data point. In our system, the number of gate openings which the signal processor can process is limited to 80, hence the maximum range span which can be measured at any one time is 23.7 km. Therefore, Figs. 8(a) and 8(b) correspond to ranges from 10 km to 33.7 km. The detectability, which is plotted on the primary vertical axis, is a form of signal to noise ratio in the spectral domain which is defined as the ratio of the signal peak and the standard deviation of the noise. It is shown that the number of random outliers in the measured LOS velocity is very small even at a range of more than 30 km. The LOS velocity at the longest range (33.7 km) is plotted for each data point in Fig. 8(c). The number of outliers in the 100 consecutive data points is 10 (i.e., 90% detection probability). It is seen that the long range wind sensing data presented in  are not the only data collected, and that stable performance can now be confirmed.
4.2.2 Vertical path wind sensing
We have in addition demonstrated vertical path measurement with a beam elevation angle of 60 degrees. This elevation angle was set using a mirror. The range dependence of the detectability and line of sight (LOS) velocity for this vertical path measurement are shown in Figs. 9(a) and 9(b). The 50 consecutive data points so obtained are plotted in these figures. The measurement height is easily calculated by multiplying the range by the cosine of 60 degrees. The range resolution is the same as in Figs. 8(a) and 8(b) (300 m), but the number of accumulated pulses is 32,000. In Fig. 9(a), the detectability decreases steeply with increasing range (height). It is considered that this is due to the steep reduction of the aerosol backscatter coefficient around the boundary layer height. It can be seen that wind sensing has been performed successfully for a height of up 7 km with only a few random outliers, while the increase in detectability at around 5 km is possibly due to the thin clouds around at the time.
We have demonstrated a high-gain, high-average-power laser amplifier using an Er,Yb:glass planar waveguide, which is suitable for a CDL. The average power of 12.8 W was achieved with a repetition frequency of 4 kHz (3.2 mJ/pulse), a peak power of 5.5 kW, and a pulse duration of 579 ns. The figure of merit (FOM) for a wind sensing CDL is basically expressed as the product of the pulse energy and square root of the repetition frequency when the pulse duration is greater than a few hundreds of nanoseconds. To the best of our knowledge, the results presented in this paper show the highest FOM for a 1.55 μm CDL which demonstrated wind sensing over a long range of more than a few tens of kilometers. The well-suppressed ASE and nearly-single-mode beam quality also contribute to long range wind sensing by the CDL. Long range wind sensing at more than 30 km for horizontal path was demonstrated with a CDL using the planar waveguide amplifier described in this paper. This amplifier has also been installed in an airborne wind sensing CDL, and long range wind sensing at high altitudes has been demonstrated , although these results are not included in this paper.
To further advance the amplifier technology of this paper, we have under development planar waveguide amplifiers with a variety of configurations, including different signal beam paths and waveguide sizes, although we have not described these developments in this paper. We recently demonstrated higher output pulse energies (7.4 mJ/pulse and 9 mJ/pulse) [62,63], and also the first demonstration of a CDL using this amplifier with a short measurable range of up to just a few kilometers . We also presented a compact planar waveguide amplifier and demonstrated the output performance of the first continuous wave laser . Future papers on this topic await detailed evaluation and development of the subject matter.
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