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Abstract
We proposed and demonstrated an all-fiber scheme for optical spectrum measurement based on stimulated Brillouin scattering and frequency self-sweeping laser without external driver and frequency tunable elements. The resolution and measuring range of proposed analyzer is measured to be 23 MHz and 5 THz respectively. The ways for improvement of the device characteristics are discussed.
© 2017 Optical Society of America
1. Introduction
Optical spectrum is one of the most important characteristics of laser radiation. A lot of techniques for spectral measurement have been already proposed and developed. The optical spectrum analyzer (OSA) with spectral resolution on the order of 1 GHz based on diffraction grating is a most commonly used spectral tool [1]. The measuring spectral range of the tool can be as wide as several hundreds of THz. A higher spectral resolution can be obtained with heterodyne technique [2]. In this case optical spectral analysis is replaced by radio frequency one and interference between a signal under test and a local oscillator is analyzed. The spectral resolution is defined by spectral properties of local oscillator and can be improved to kHz level. For classical heterodyne technique, the measuring spectral range is limited by the bandwidth of photodetectors, which does not exceed several tens of GHz. For ultra-wideband optical systems, a tunable single frequency local oscillator is used. In this case, the measuring spectral range can be increased up to several tens of THz. Moreover, a special type of heterodyne technique allows one to analyze spectral evolution with nanosecond timescale [3]. High-resolution OSAs based on stimulated Brillouin scattering (SBS) amplification were proposed and demonstrated [4,5] more than ten years ago. Narrowband pump wave and a signal under test counter-propagate along an optical fiber in this type of OSA. The signal radiation is amplified via SBS within relatively narrow bandwidth of the Brillouin gain window when frequency difference between pump wave and the signal under test corresponds to Brillouin shift of the fiber. Spectrum of the signal under test can be reconstructed from measurements of the signal amplification versus pump wave frequency. The bandwidth of the Brillouin gain window, which is order of several tens of MHz, determines spectral resolution of the technique. Some special methods are applied to improve the resolution up to several tens of kHz [6]. The measuring spectral range of SBS-based OSA is limited by sweeping range of tunable pump laser and reaches up to ~10 THz. Narrowband laser with broad tuning range is an essential part of the SBS-based as well as heterodyne techniques, operating with tunable local oscillator. Such sources are rather complex and, as a result, expensive. A self-sweeping fiber laser [7,8] can be an alternative tunable source.
The self-induced laser frequency sweeping (self-sweeping for short) effect consists in periodic dynamics of the laser frequency. The main mechanism of frequency change is the influence of dynamically recorded gain and phase gratings on competition between the laser modes [8]. As it was shown in [8] the laser, having certain cavity configuration, generates periodic sequence of microseconds pulses containing single longitudinal mode radiation only. At the same time, the laser frequency is changed from one pulse to another by one or several intermode beating frequencies. Due to broad sweeping range (more than 20 nm [9]) and simplicity, the self-sweeping fiber lasers are attractive sources for applications demanding tunable radiation. In particular, single-frequency self-sweeping ytterbium-doped fiber laser was applied for high-resolution characterization of optical elements [10,11] and fiber sensor interrogation [12].
Here we report on the experimental demonstration of SBS-based technique of high-resolution characterization of laser spectra in all-fiber configuration. The key element of the proposed OSA is the self-sweeping fiber laser operating without frequency tuning elements. The SBS-based OSA has been successfully tested to measure the spectra of multi-frequency as well as single-frequency lasers. The spectral measurements are compared with results obtained with standard grating-based OSA. The resolution of proposed OSA is measured to be about 23 MHz, which corresponds to bandwidth of the Brillouin gain window for used fiber. Finally, we discuss some ways for further improvement of the proposed scheme.
2. The basic principles and theory
Let’s consider the basic principle of operation of the SBS-based OSA. A CW narrowband pump wave with power of and wavelength of is launched into passive fiber with total length of at . Signal from laser under test (LUT) with initial power is launched at from the opposite side of the fiber. The signal wave is amplified at Stokes wavelength within the bandwidth of the Brillouin gain window. Therefore, for the sake of simplicity the initial signal under test at can be divided into two spectral parts, corresponding to in and out of resonance: . The second (out of resonance) term can be comparable with the first (in resonance) one in the case when the width of initial spectrum is comparable with the bandwidth of the Brillion gain. If propagation losses can be neglected due to small length of the fiber, then the input and output resonance parts of the signal are related by an equation [13]:
where , , is a measure of the SBS efficiency as it shows fraction of the input pump power , which is converted to the Stokes power , is the small-signal gain associated with the SBS process, is the peak value of the Brillouin gain coefficient. Relation between input and output resonance signals becomes simple when SBS efficiency is low ( <<1):The measured output signal can be expressed in this case as a linear combination of resonant and nonresonant terms:
Unamplified spectral signal components being outside the SBS gain window propagate to the output with amplified components and thereby deteriorate the dynamic range. This fact complicates methods applied for detection of a weak spectral component in the presence of a strong second one at a closely spaced frequency. The unamplified signal can be suppressed or taken into account in the signal processing. The one way to suppress the influence of unamplified signal is application of the technique of polarization pulling (see [6] and reference therein). The method is based on the fact that polarization state of the amplified signal is different form polarization state of unamplified signal at certain conditions in standard weakly birefringent fibers. Therefore, an output polarizer and fine-tuning of polarization states of pump and signal wave is required for implementation of the technique. In the paper, we propose the way of accounting unamplified signal in the signal processing. For this purpose, additionally it is required to measure signal when pump power is off ():
Then the difference between the two measured values
is proportional to the signal wave power at the resonance wavelength . Thus, tuning the pump wavelength and measuring transmitted signal power increase will allow one to evaluate spectral shape of the signal wave. One should note, that if SBS efficiency is high (~1), then evaluation of the signal spectra become much more complicated.3. Experiment
The SBS-based OSA consists of the self-sweeping laser and SBS amplifier [Fig. 1]. The scheme of all-fiber polarization maintaining (PM) Yb-doped self-sweeping fiber laser used as tunable pump laser with passive amplitude modulation is shown in Fig. 1(a). PM Yb-doped double-clad fiber (Nufern PM-YDF-5/130) with length of 3 m is used as a gain medium in the laser cavity. The fiber is pumped through a pump combiner by a multimode laser diode with power up to 9 W. A fiber loop mirror based on polarizing 50/50 PM coupler is used as highly reflective mirror. A proper choice of passive fiber length (~10 m), located between the pump combiner and the fiber loop mirror, ensures self-sweeping regime with generation of periodic microsecond single-frequency pulses [lower red traces in Figs. 2(a)-2(c)]. Each pulse contains practically only single longitudinal mode radiation, and the laser frequency is changed between the pulses by one intermode beating frequency (~5.67 MHz). One should note that instantaneous linewidth of the laser (about 1 MHz) is considerably narrower than bandwidth of the Brillouin gain profile, which ensures efficiency of SBS process in SBS-based OSA. A 20/80 PM filter coupler with a PM isolator at 80% output port and right angle cleave at the other output port is used as an output coupler in the low Q-factor laser cavity. At operating pump power of 2 W the laser generates in self-sweeping regime with wavelength scanning range of 1050-1070 nm at rate of ~1nm/sec. Average and peak powers measured at output port of the isolator amounts to ~100 and ~400 mW respectively.
The scheme of SBS amplifier is built with PM components and consists of two 95/5 PM couplers and a short spool of PM fiber (Nufern, PM980XP, the Brillouin frequency shift is about 15.9 GHz) with length of 30 m [Fig. 1(b)]. The tunable attenuator is used to control power level of a LUT. The first input end is used for pump laser and the second one for the LUT. The pump laser and LUT are linearly polarized along slow axis of the PM fiber to maximize SBS amplification. The signal from the pump laser and amplified radiation from LUT are detected with two fast detectors with bandwidth of 5 GHz (Thorlabs, DET08CFC/M) and then sampled with an oscilloscope (LeCroy, WavePro 725Zi-A). It should be noted that there is no SBS of pump pulses without LUT radiation.
Two types of lasers are used as LUT. The first LUT is homemade linearly-polarized Yb-doped fiber laser [Fig. 1(c)]. Yb-doped double-clad fiber (Nufern PM-YDF-5/130) with length of 9 m is used as a gain medium in a laser cavity. The fiber is pumped through a pump combiner by multimode laser diode with pump power up to 18 W. The laser cavity is formed by a fiber loop mirror based on 50/50 fused PM coupler at one cavity end and a low-reflection output FBG with reflection coefficient of ~27%, at central wavelength of 1064 nm with FWHM bandwidth of 70 pm inscribed in PM fiber at the other end. The FBG is placed in a thermostat and its wavelength is temperature controlled. A 2 m long piece of polarizing fiber (HB-1060-Z 7/125) is used for polarization selection in the first LUT. An additional 99/1 PM coupler is used for intracavity spectra measurements. The total length of the homemade Yb-doped fiber laser is about 20 m. The second LUT is a fiber-coupled commercially-available linearly-polarized single-frequency Nd:YAG laser (Mephisto, InnoLight) with central wavelength of 1064 nm and linewidth of ~1 kHz. The second LUT is used for instrumental function measurements of the SBS-based OSA and as a wavelength reference source.
At first, we measured signals with fast photodetectors with bandwidth of about 5 GHz at standard load resistance of 50 Ω [Figs. 2(a)-2(c)]. The output signals are sufficiently noisy due to fluctuations of peak power of pump laser and of the first LUT [Fig. 2(b)]. Since bandwidth of the Brillouin gain window is about 20 MHz and intermode beating frequency of the first LUT is about 5 MHz, each pump pulse amplifies several longitudinal modes in Yb-doped LUT simultaneously. Furthermore, the mode dynamics can be rather complex (see, for example [14],). As a result, the amplified signal consists of many narrowband peaks originating from interference of longitudinal modes with random phases [Fig. 2(c)]. Because of small number of interfering modes amplifying within Brillouin gain window one can see in the Fig. 2(c) some regular structure with period of 200 ns, corresponding to roundtrip time in the LUT cavity. The amplified signal contains some important information about mode dynamics in the LUT, but one need to measure averaged spectra usually. The detector’s bandwidth was reduced with an additional load resistor to integrate power and smooth these fluctuations. We have restricted detector’s fall-time to about 5 µs because the time should be limited by repetition period of the pump pulses (~8-20 µs) of the self-sweeping laser. The measured signals with optimal load resistance of ~20 kΩ (detector’s bandwidth ~10 MHz) are presented in Figs. 2(d)-2(f). On the one hand, the signal is less noisy in this case than in the case of 50 Ω load resistance; on the other hand, measured signal between pump pulses goes to zero [Fig. 2(f)] which allows one to measure the unamplified signal level in the absence of pumping.
It was pointed out in the theoretical section, that one need to keep Brillouin amplification efficiency for a LUT at low level. We fixed power of tunable pump laser and varied the power of signal laser during the experiments. At rather high level of injected signal power, the effect of pump depletion was observed. It was also found during the preliminary experiments that at pump power of ~100 mW linear growth of output versus input signal power is observed, when injected level of LUT power is below 0.5 mW. It should be also noted that this value depends on peak pump power as well as on the fiber length.
After selection of the detectors bandwidth and suitable power level of signal laser, we measured spectra of two laser sources (Yb-doped fiber laser and single-frequency Nd:YAG laser) simultaneously. To eliminate unamplified radiation from desired signal the following procedure was performed. We measured time dependence of power for pump laser and amplified radiation under test simultaneously. Moments in time corresponding to maximum (pump is on) and minimum (pump is off) level of pump power were determined from the former dependence. Using these positions in time for the later time dependence one can find signal power increase subtracting signal power level at maximum pump from the signal level when pump is off. It should be noted that each point in the obtained data corresponds to single-frequency pump radiation and frequency change between points is equal to one intermode beating frequency for self-sweeping laser (~5.67 MHz). The peak position of single-frequency LUT is set as zero frequency detuning. Additional averaging over five measurements is applied to reduce noise in Yb-doped fiber LUT spectrum, which as discussed earlier is caused by short term mode fluctuations in the LUT. Then the integral of spectral function is normalized to total output signal power.
The typical spectrum of two lasers measured with SBS-based OSA is presented in Fig. 3 by red lines. For comparison, the spectrum was measured with grating-based OSA (Yokogawa, AQ6370) at highest spectral resolution of ~2 GHz (thick black line in Fig. 3). From the Fig. 3 one can see, that the spectra are qualitatively agree with each other. Most of the power is concentrated within relatively broad Yb-doped fiber LUT laser line. The both type of OSA demonstrate similar spectral line shape here, but the line shape measured with SBS-based OSA is more symmetrical due to narrower instrumental spectral function of the OSA. Nd:YAG laser generating near zero detuning has spectral linewidth of 1 kHz. The both type of OSA demonstrate instrumental spectral function here. Integral over the instrumental function corresponds to power, which is generated by the Nd:YAG laser. Thus, ratio of the peak amplitudes (~20 dB) corresponds to inverse ratio of instrumental function widths of the OSAs. The conclusion is in agreement with SBS-based OSA spectral linewidth measurement of 23 ± 1 MHz [Fig. 3(b)]. The noise level for SBS-based OSA is higher than for grating-based one. Signal to noise ratio for SBS-based OSA with self-sweeping laser is estimated as 20 dB. The additional averaging over adjacent spectral region and/or any type of smoothing lead to noise reduction. However, the last procedure results in degradation of spectral resolution.
Fig. 3 The typical spectrum of lasers under test measured with SBS-based (red line) and grating-based (thick black line) OSAs (a). (b) is a zoomed view of Nd:YAG spectrum (a).
4. Conclusion
In the paper, we proposed an all-fiber scheme for optical spectrum measurements based on SBS and self-sweeping fiber laser. Resolution of the proposed OSA is measured to be 23 ± 1 MHz, which can be improved by selection of another fiber type with reduced width of Brillouin gain. The measuring range for demonstrated OSA is rather large (~5 THz) which can be extended up to at least 7.5 THz [9]. Since the total fiber length in the OSA was about 30 meters, the gain factor was low and as a result value of signal-to-noise ratio is also rather low (~20 dB). Moreover, we used standard biased photodetectors with rather high noise level. Usually for SBS-based OSA single-mode fibers with length of several km are used. As a result, the dynamic range of the SBS-based OSA exceeds 60 dB. Therefore, the optimization of fiber length as well as detection scheme is required for increasing dynamic range. The linearly-polarized pump radiation and all-PM scheme eliminate the need for polarization controllers to match polarization states of waves. The demonstrated SBS-based OSA operates in the spectral range of 1 um, which is determined by spectral range of the self-sweeping laser. The similar OSA can be built for some other spectral regions using other self-sweeping lasers such as bismuth [15], holmium [16] or thulium-holmium [17] fiber lasers.
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