Mahmood Bagheri, Clifford Frez, and Siamak Forouhar
Jet Propulsion Laboratory, California Institute of Technology, U.S.
Abstract.
We report on the first fiber-coupled distributed-feedback (DFB) semiconductor lasers with record high output power operating near 2.051 mm wavelength. The developed single-mode, fiber-coupled lasers deliver more than 35 mW output power from polarization maintaining optical fibers with less than 200 KHz intrinsic linewidth and better than 40 dB side-mode suppression. This is an order of magnitude improvement in output optical power and laser linewidth compared to the state-of-the-art semiconductor lasers operating at this range, and will pave the way for the implementation of semiconductor diode lasers in injection seeding applications for high-power lidar transmitters.
The wavelength range around 2.0 mm has become a spectral region of interest in the Earth science community due to its use in 3-D wind measurement systems as well as profiling water vapor and carbon dioxide (CO2) concentration studies [1]. Currently, the transmitter architecture in these atmospheric sensing applications uses high-power, frequency-locked, solid-state seed lasers to injection seed high energy optical pulses from a slave laser. The system’s complexity and reliability could be substantially improved if solid-state seed lasers could be replaced with monolithic, fiber-pigtailed semiconductor lasers operating at this wavelength range. While fiber-based injection seeding sources (such as DFB diode lasers) are commercially available at the telecommunication band (1.5 mm range), lack of high output power and stable optical sources beyond 2 mm has hindered probing atmospheric tracers (such as CO2) at mid-infrared range where tracers have stronger absorption lines.
Currently, CO2 concentration profiling lidar operating at 2.05 mm range require fiber-pigtailed single-mode seed lasers with at least 30 mW continuous wave (CW) optical power and better than 1 MHz frequency jitter to accurately resolve CO2 absorption lines near 2.05 mm. To date, state-of-the-art diode lasers operating single mode near 2 μm are limited to output powers near 5 mW [2]. In order to fill this gap, we have developed fiber-coupled, high-output, narrow linewidth 2.05 mm diode lasers based on our well-established, laterally coupled distributed feedback (DFB) semiconductor laser design [3]. The fiber-pigtailed semiconductor laser modules operate single-mode near 2.051 mm wavelength with an unprecedented 35 mW output power near room temperature. The fiber pigtailed lasers deliver laser power in polarization, maintaining optical fiber with excellent side mode suppression ratios and a mode-hop free tuneability of larger than 1 nm. The polarization maintaining (PM) output optical fibers improve the slave laser stability by eliminating polarization drifts.
The laser modules incorporate DFB lasers fabricated from a wafer with four InGaAsSb quantum wells centered within AlGaAsSb waveguide and cladding layers, grown on a GaSb substrate by molecular beam epitaxy. Second-order grating were patterned on the periphery of a 4 mm wide ridge to select a single longitudinal mode at the designed wavelength. The laser design and fabrication procedures have been published elsewhere and will not be covered here [3]. After fabrication and laser facet coating steps, laser chips are cleaved into individual 2-mm long laser bars, and are mounted on gold-plated AlN submounts and then packaged with a PM1950 Nufern optical fiber (polarization maintain optical fiber with transmission between 1850-2200 nm). The optical elements are anti-reflection coated to minimize optical feedback into the laser cavity and avoid potential instabilities and linewidth broadening. The PM fiber tip collecting optical power inside the package is slightly angled to further suppress reflections into the laser. Figure 1 shows a schematic representation of the laser packaging unit.
Figure 1. (a) The left axis shows CW output power collected from the end of optical fiber. The right axis shows the corresponding voltage across the diode PN junction. (b, c) Lasing spectra measured between 250 mA- 410 mA (b) and 450 mA-530 mA (c). (d) Lasing wavelength tuning versus injection current at different heat-sink temperatures. Image Credit: M. Bagheri.
Figure 1(a) shows the CW output optical power collected from the end of optical fiber. Measurements were taken at 10 degreesCelsius heat-sink temperature, and a calibrated thermopile detector was used to collect output power. The lasers show a threshold current density of 450 Acm-2, indicating the high quality of material growth and device fabrication. The slope efficiencies from the emitting laser facet and the end of the optical fiber is 0.074 mWmA-1. Figure 1(b, c) shows output optical spectra collected from the optical fiber end measured at different injection currents at 10 degree C heat-sink temperature. The DFB lasers show a single mode behavior over a large current range. Figure 1(d) shows the lasing wavelength at different injection current and heat-sink temperature. The lasers exhibit stable and constant wavelength (l) tuning by dldT-1 = 0.18 nmK-1 over a large current and temperature range. The large mode-hop, free tuning range of these lasers makes a good candidate for absorption spectroscopy instruments, as they can span over many trace gases absorption lines (e.g., CO2).
Figure 2(a) shows the measured frequency noise spectrum of the laser module above lasing threshold. The white noise region of the spectra reveals a Lorentzian linewidth of less than 100 kHz at 30 mW. The spurious peaks at 15 kHz and below are due to mechanical resonances from the Fabry-Perot interferomter used for converting phase noise into amplitude noise [4, 5]. The laser lineshape with contributions from both 1/f-noise and white noise contributions is a Voigt profile, with a total linewidth of 700 kHz at 30 mW over 5 ms period of observation.
Figure 2. Measured frequency noise spectrum of the laser module above lasing threshold. Image Credit: M. Bagheri.
In summary, we have demonstrated high-power, fiber-pigtailed, distributed feedback (DFB) semiconductor lasers at 2.05 mm wavelength. The laser output power exceeds 30 mW at the end of the optical fiber. The lasers show excellent side-mode suppression ratios and mode-hop free wavelength tuning over a large current and temperature range. The high output power fiber-pigtailed and narrow linewidth semiconductor lasers at 2.05 mm range, along with the recent progress in development of optical fiber amplifiers at this wavelength, enable novel applications and instruments such as CO2 concentration measurements in air and open up the possibility of long-haul optical communication using multi-mode photonic band gap optical fibers. This demonstration is an important step toward deployment of these lasers as injection seed lasers for airborne and spaceborne lidar systems for CO2 detection.
Acknowledgement
This work was supported under the Advanced Component Technology program of NASA’s Earth and Science Technology Office and performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA.
References
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