Photonic Crystal Fibers for High-Performance LIDAR Applications
Abstract: We explore photonic crystal fibers (PCF) to develop new enabling systems for high-performance LIDAR. Development of new fiber technologies with high-power scalability is crucial for making efficient, compact and robust light sources practical for a wide variety of LIDAR applications including environmental, scientific, and military. Given, especially the current atmospheric and geological trends in our planet, more accurate and high-precision monitoring and detection of our environment has become rather significant. PCF based lasers and amplifiers are highly attractive in the sense that they can offer solutions superior to traditional fiber technologies. They have already produced an immense literature in a very short time, ranging from hollow-core fibers to supercontinuum generation, frequency comb, spectroscopy and fiber amplifiers, among others.
Keywords: Photonic Crystal Fibers, LIDAR
Collaborators: Nan Yu (NASA)
Wildfires have charred about 460,000 acres this week (at the time of writing) in Southern California [1]. As it has been recently reported in Science by A. L. Westerling, et al. [2] wildfires become increasingly more common in Western US due to warming trends and other climate factors. Authors noted that average length of the fire season between 1987-2003 was 64% longer than it was between 1970-1986. This suggests more observations for weather prediction and climate monitoring with increased precision and accuracy. Proposed instruments would measure water vapor, solar radiation, sea surface winds, cloud activity, and land cover, among others [3].
When Hurricane Katrina hit New Orleans in August 2005, floodwaters resulted in infrastructure failures followed by many hundreds of fatalities. The evacuation decision based on observations made from Earth-sensing satellites helped avoid the much worse situation [3].
Light detection and ranging (LIDAR) systems, in contrast to microwave radars operate in higher frequencies and this leads to higher accuracy and resolution. Atmospheric propagation, however, limits their performance. Efficient, compact, robust, high power, and high energy light sources are essential not only for reliable remote atmospheric sensing and imaging applications, but also for reliable airborne LIDAR for military tactical range and velocity systems, and aircraft navigation. In this regard, pulsed fiber lasers and amplifiers [4-7] with multi-kHz repetition rate (i.e., higher than bulk solid state Q-switched lasers), ns pulses of narrow linewidth, and high signal to noise ratio (SNR) are of special interest for environmental, scientific and military applications thanks to their spectral and spatial qualities as well as compactness, while maintaining the optical performance for above applications.
Photonic crystal fibers (PCFs) have created an immense literature since mid 1990s. Besides the fundamental physics, many interesting applications have been proposed. Supercontinuum generation, frequency comb, spectroscopy, fiber lasers and amplifiers, sensors, quantum correlated photon pairs, high-energy transmission are to name few.
In this project we explore PCFs to develop new enabling systems for enhanced airborne LIDAR and remote sensing/imaging applications especially for environmental observing and monitoring. Below, we give a brief overview of the light source requirements for high-performance LIDAR systems and introduce PCFs.
Requirements for High-Performance LIDARs
Water vapor and CO2 are the main contributers to spectral absorption in the near infrared band (800nm-2500nm). In this region commercial Yb, Er-Yb and Tm silica fiber lasers and amplifiers are available. Yb lasers operate in 1000nm-1060nm range, while Er-Yb and Tm in 1480nm-1620nm and 1800nm-2100nm ranges, respectively.
Figure 1 Block diagram of a simple MOFA system for coherent LIDAR [6].
The ability of the LIDAR to detect low level signals in the presence of noise can be quantified by the ratio of singal power to the noise power. Coherent LIDARs employ heterodyne detection to mix or interfere the scattered field from the target with a local oscillator. Figure 1 illustrates a simple MOFA configuration to achieve this task. There has to be perfect phase matching between the detected field and local oscillator. This provides various useful information such as reflectivity from signal strength, range from time of flight, speed from frequency or phase shift, and target composition from polarization. Because LIDAR efficieny can be affected from atmospheric turbulence especially for long range and large apertures, short wavelengths are less desired. Atmospheric aerosol transmission increases with wavelength. Only reflectance favors shorter wavelengths for high SNR.
Consider, for example, a pulsed Doppler wind LIDAR [6] for atmospheric remote sensing. Here, SNR gives better range perfomance with increasing wavelength and pulse length in the expense of accuracy and resolution. For a given range resolution, pulse energy can be traded for the repetition rate with an identical pulse length.
Photonic Crystal Fibers Paradigm
There are many scientists and engineers working to improve various features of fibers. They investigate fibers with more versatility that could carry more power and easy to use for sensing; fibers with multiple cores, higher or lower nonlinearities, higher birefringence, more thermal stability, and flexibility in dispersion engineering, among others.
There are two main factors limiting the versatility of conventional fibers [8]. One, small core-cladding index contrast, (less than 1% in telecom fiber) which lacks the ability of manipulating dispersion and birefringence on demand. Two, the reliance on total internal reflection which doesn't allow air-guidance in a hollow core. This would, however, be useful for reduced nonlinearities and enhanced laser interactions with dilute or gaseous media.
In 1995, T. A. Birks, et al. [9] from Southampton University demonstrated a new type of optical fiber which shows full 2D bandgaps for all polarizations in silica/air. Same group, in 1997, built an all-silica PCF which shows single mode behaviour at all wavelengths within the transparency window of silica [10]. J. C. Knight, et al. [11], in 1998, reported the realization of large mode area single mode fiber with a core diameter of fifty free-space wavelengths. In 1999, R. F. Cregan, et al. [12] reported in Science a hollow-core PCF which allows single mode photonic bandgap guidance of light in air. Single mode hollow-core PCFs have potential for ultra-high-power transmission applications in vacuum. A. Ortigosa-Blanch, et al. [13] reported in 2000, highly birefringent polarization maintaining PCF operating at telecom band. In the same year first Yb-doped PCF laser (W. J. Wadsworth, et al. [14]) and white-light supercontinuum generation (K. R. Jinendra, et al. [15]), extending from infrared to the violet (i.e., 550THz width), were also reported. The latter, 10,000 times brighter than the Sun, was generated in PCF with zero dispersion at 800nm, pumped by 150fs Ti-Sapphire pulses of energy ~1nJ. First experimental demonstration of nondegenerate four-wave mixing and quantum-correlated photon pair were reported by the Northwestern group (J. E. Sharping, et al.) in 2001 [16] and 2004 [17], respectively. G. Kakarantzas, et al. [18] reported in 2003, highly compact, temperature stable, efficient rocking filters in polarization maintaining PCFs. In 2005, A. H. Al-Janabi and E. Wintner reported a high-power laser transmission through hollow-core PCF [19].
Hollow-core PCF has many promising applications. High-efficiency gas-Raman cells, low-threshold frequency conversion of laser light, laser-tweezer guidance of small particles are to name a few. Optical sensing is another relatively unexplored area which may provide myriad opportunities ranging from environmental monitoring and detection, and biomedical sensing to structural monitoring.
Current state of art in PCF technology has proved following striking improvements over the prior art [8]:
* Seven orders of magnitude improvement over nonlinear gas-laser devices.
* More than octave-wide supercontinuum sources five orders of magnitude brighter than incandescent lamp.
* 1ps white-light pulses 10,000 times brighter than Sun.
* Unprecedented frequency measurement accuracy using an octave spanning fs frequency comb. This helped Theodore Hansch to win a Nobel prize.
* Three orders of magnitude adjustable fiber nonlinearity.
* 10 times higher birefringence and 100 times more temperature stability compared to conventional approaches.
Figure 2 MOFA system that was used in Ref. [5]. Yb-doped PCF forms the main part of the architecture. MO: Master oscillator, OI: Optical isolator, L: Lens, YDF: Yb-doped standard fiber, LPF: Dichroic long-pass filter (~98% transmission at 1062nm signal wavelength, ~98% reflection at 976nm pump wavelength), BPF: Optical band-pass filter.
F. Di Teodoro and C. D. Brooks [5] have recently reported diffraction limited and narrow linewidth (~9GHz) pulses using a MOFA system (Figure 2). MOFA architecture allows temporal (i.e., repetition rate, pulse shape and duration) and spectral (i.e., linewidth and wavelength tunability) control for a pulsed narrow-linewidth source. They obtained ns pulses of energy >1mJ, ~1.1MW peak and ~10.2W average power. A large core (40um diameter) Yb-doped, single mode PCF amplifier was the main part of their MOFA architecture. Master oscillator that they have employed operates in the Q-switch regime producing 1ns pulses at 9.6kHz repetition rate and 1062nm wavelength.
Despite the fact that fibers with core diameter about 30um are highly multimode, large-core fibers can be made to operate on the fundamental mode with appropriate modal excitation and by suppressing higher-order coupling [4].
Nonlinear effects become significant in the MOFA especially for high peak power pulses associated with short duration and high energy. This, in general, results in four-wave mixing, stimulated Raman scattering, and self-phase modulation [4-5]. They can detoriate both temporal and spectral properties of the pulse. The output spectrum of Ref. [5], however, shows excellent signal where all other spectral features including amplified spontaneous emission are 55dB below the signal level. Only the central mode contributes to useful signal. Given the overall spectral fidelity, large core PCFs appear to be very beneficial in minimizing nonlinear effects and storing more energy [4-6]. Despite the high SNR, however, the amplified pulses are not Fourier-limited and five times broader than the near-Fourier-limited pulses generated by the master oscillator.
Fourier-limited pulses with improved spectral purity can be obtained by exploiting high concentration, shorter and larger-core PCFs due to further minimization of residual nonlinear processes such as self-phase modulation [4-5]. Because the nonlinear effects scale with the intensity-length product. Additionally, further scaling of pulse energy and peak power can be explored.
Conclusion
Given the current atmospheric and geological trends in our planet, more accurate and high-precision monitoring and detection of our environment has become more significant. Therefore, development of new eye-safe fiber technologies with high-power scalability is crucial for making efficient, compact and robust light sources practical. To achieve this task, PCF technology can be highly attractive paradigm. PCFs and fiber lasers have been the two of the most rapidly evolving fields of optics and photonics for the past few years [20]. Recent developments in this field have enabled new ways of generating, transforming, and delivering light, which have been impossible with conventional fiber optics. PCFs may significantly affect the development and deployment of fiber laser technologies in the years to come.
References
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