A simple and low-cost 1550 nm semiconductor laser with subkilohertz intrinsic linewidth is experimentally demonstrated. A commercial distributed feedback diode laser is self-injection locked to the resonance transmission peaks of a fiber Bragg grating Fabry-Perot cavity through a polarization-maintaining fiber ring with the optical path length of 4 m, with the laser frequency noise suppressed by over 70 dB in the Fourier frequency band from 5 Hz and 1 kHz. The laser features an intrinsic Lorentzian linewidth of 125 Hz as well as a relative intensity noise of <–142 dBc/Hz above 2 MHz, and provides over 0.8 nm quasi-continuous tunability, which is suitable for advanced applications requiring a narrow linewidth laser with ultralow frequency noise.
© 2016 Optical Society of America
Narrow linewidth laser sources with low-frequency noise are key tools in a broad range of applications such as high-resolution spectroscopy , interferometric fiber optic sensing , coherent optical spectrum analyzer , and phase coherent laser communication . In most instances, both fiber distributed feedback lasers and non-planar ring oscillator (NPRO) solid state lasers are available as options for fulfilling the noise and linewidth requirements. However, their intrinsic high cost, low efficiency and bulky size make both of these lasers less suitable for many practical implementations [5,6]. In recent years, numerous methods have improved significantly to suppress the frequency noise of the diode lasers, making the diode laser attractive solutions for narrow linewidth laser sources.
Compactly designed external cavity diode laser (ECDL) modules based on dispersive components (such as volume holographic Bragg gratings , planar waveguide Bragg reflectors [2, 7] and fiber Bragg gratings [8–10]) or the high quality factor (Q) resonators [5,11,12] offer superior performance relative to that of conventional distributed feedback (DFB) and distributed Bragg reflector (DBR) diode lasers, whose typical linewidths are a few hundred kilohertz to a few ten megahertz. In particular, mature butterfly packaged products of the narrow-linewidth ECDLs have appeared on the market, providing extraordinary high stability and low noise. Redfern Integrated Optics Inc. has designed planar-waveguide ECDLs to operate under harsh environmental conditions, which is composed of a semiconductor gain chip and a planar lightwave circuit waveguide with Bragg grating and demonstrated a fitted Lorentzian full width at half maximum (FWHM) linewidth less than 1 kHz . OEwaves Inc. has demonstrated a 30 Hz integral linewidth and a sub-Hz intrinsic Lorentzian linewidth in their recent chip-scale heterogeneous integrated systems by self-injection locking a DFB diode laser to a crystalline whispering gallery mode (WGM) micro-resonator . Although these techniques can offer dense integration solutions with a very narrow linewidth, they still need complicated and time-consuming fabricating procedures or expensive optical components that may not be suitable for cost-sensitive applications. Consequently, a major effort has also been directed to obtaining the equivalent performance using an active electrical servo-loop and a frequency discriminator. The all-fiber frequency reference with a flexible filtering spectrum, no need of fine alignment and tight polarization adjustment, and spatial mode matching, making it a prime choice for largely lighter, more compact and cheaper frequency discriminators . By locking to the sharp transmission notch of a π-phase shifted Bragg grating, the linewidth of a commercial DFB diode laser can be reduced to 2 kHz . Nevertheless, the high-frequency noise of the actively stabilized lasers is usually not substantially reduced due to limited electrical servo-loop gain at high frequencies. Although the high-frequency noise can be suppressed by additional weak optical feedback from a long external cavity implemented with an optical polarization-maintaining (PM) fiber, it also increases the complexity of the system to some extent [15,16].
In this paper, we report a commercial DFB diode laser self-injection locked to resonance transmission peaks of a fiber Bragg grating (FBG) Fabry-Perot (FP) cavity with a Q-factor of approximately 6.5 × 106. A small portion of the FBG-FP cavity transmission light is injected to the DFB diode laser through PM fiber for resonant optical feedback. Owing to the intrinsic high frequency sensitivity of the diode laser to the injection light and the high Q-factor characteristics of the FBG-FP cavity, a drastic reduction of the phase/frequency noise is realized which narrows the laser linewidth. Furthermore, the external cavity with an all-fiber configuration makes it easy to extend the cavity length, which contributes to further suppress the phase/frequency noise at high frequencies. Benefiting from this technique, the system exhibits a noise suppression of over 70 dB with the Fourier frequencies between 5 Hz and 1 kHz, and the integral laser linewidth is reduced to around subkilohertz. Moreover, it keeps the white noise plateau as low as 40 Hz2/Hz above the Fourier frequencies of 1 kHz, featuring a 125 Hz intrinsic Lorentzian linewidth. A stable operation has been acquired without any additional electronic feedback. To our best knowledge, although the FBG-FP cavities commonly have served as a narrow bandwidth bandpass filter to select single-longitudinal-mode (SLM) in fiber ring laser [17,18], this is the first report to use it as a frequency discriminator to directly suppress the frequency noise of the DFB diode laser using resonant optical feedback.
2. Experimental setup
The schematic diagram of the experimental setup is shown in Fig. 1, and the inset visualizes the DFB diode laser self-injection locked to different resonance transmission peaks of the FBG-FP cavity transmission spectrum. The laser source is a single-mode DFB-type diode laser at 1550 nm for communication, which is PM fiber-coupled and butterfly-packaged without an internal isolator. It is driven by an ultralow noise current source (ILX Lightwave, LDX-3620B) and its temperature is stabilized by a precision temperature controller (ILX Lightwave, LDT-5948). The FBG-FP cavity used in the experiment is fabricated in the standard single-mode fiber (SMF-28) core, and it consists of two 6 mm long identical FBGs separated by a center-to-center distance of 20 mm. The transmission spectrum of this FBG-FP composes of a comb of resonance transmission peaks, as illustrated by curve (a) in the inset of Fig. 1.
The resonant optical feedback from the FBG-FP cavity transmission light is obtained by unidirectional fiber ring using a three-port PM circulator with the isolation>50 dB. Port 2 of the circulator connects to the DFB diode laser, an PM fiber output coupler (not shown in Fig. 1) is placed after the port 3 to divide the diode laser output into two equal parts, in which 50% of the diode laser’s light is used for the subsequent measurement and the rest is imported into the FBG-FP cavity with only the resonant transmission light re-injected into the DFB diode laser from port 1 of the circulator. All components along the optical train (except the FBG-FP cavity) are PM fiber-coupled and fast axis blocked to avoid laser polarization drifts. The total equivalent length of the external ring cavity is about 4 m (corresponding a longitudinal-mode spacing of 25 MHz). If the frequency of the free-running DFB diode laser is in the locking range of a certain resonance transmission peak of the cavity, the laser locks its frequency to the cavity resonance transmission peaks (as shown in curve (b) in the inset of Fig. 1); meanwhile, the DFB diode laser also acts as a longitudinal-mode selective component to avoid multiple longitudinal modes. The DFB diode laser could be tuned by locking to different transmission resonance transmission peaks of the cavity through changes in the temperature and driven current. The locking range is mainly depended on the injection light level, which can be controlled using a variable optical attenuator (VOA).
3. Experimental results and discussion
The output optical spectra of the self-injection locked DFB diode laser and the transmission spectrum of the FBG-FP cavity are measured using an optical spectrum analyzer (OSA) with a resolution of 0.04 pm (APEX Technologies, AP-2041B), as shown in Fig. 2. The gray curve in Fig. 2 is the transmission spectrum of the FBG-FP cavity composed of a comb of resonance transmission peaks with a free spectral range (FSR) of 0.036 nm, the linewidth (FWHM) of the central transmission peak of the cavity is 30 MHz at a Bragg wavelength of 1549.7 nm, corresponding a Q-factor of approximately 6.5 × 106, and the stop bandwidth of the uniform FBGs is ~0.9 nm, more than 25 transmission peaks are contained in the transmission spectrum of the FBG-FP cavity. The central resonance transmission peak (~1549.7 nm) exhibits smallest bandwidth as compared with the peaks located near the edges of the transmission spectrum due to the high reflectivity, and it should be noted that the transmission property is sensitive to the applied stretching and the polarization state of the incident light. The FBG-FP is mounted by the temperature-compensating package to reduce the influence of the laboratory environment.
The resonant transmission light is injected to the DFB diode laser through the PM circulator. To prevent instability in the DFB diode laser caused by injecting too much power, the injected ratio between the transmission light power near the desired FBG-FP cavity resonance transmission peak and the DFB diode laser free-running output power should be adjusted by a variable optical attenuator (VOA) for optimal locking. The injection ratio is measured to be more than –21.5 dB during the whole tuning process, and the laser frequency can be pulled by the FBG-FP cavity resonance frequency in a range exceeding 4 GHz. The DFB diode laser integrates an temperature controller in the butterfly-package, and its optical frequency can be tuned directly both by the operation temperature and the drive current with the tuning coefficient of ~0.1 nm/°C and ~0.007 nm/mA. Considering the high temperature tuning coefficient, coarse frequency matching is accomplished by thermal tuning of the DFB diode laser, and then further fine matching is achieved by adjusting the drive current. The optical spectra of the DFB diode laser self-injection locked to the different resonance transmission peaks are indicated by the colored curves shown in Fig. 2, which is divided into five regions according to the different operation temperature of the diode laser. At certain operation temperature, the DFB diode laser can be locked to the discrete resonance transmission peaks successively by tuning the driven currents. Likewise, for the certain drive current, the DFB diode laser can be locked at different resonance transmission peaks by changing operation temperature (see the optical spectra curves with the same color in Fig. 2). Furthermore, the DFB diode laser can also be locked to the discrete resonance transmission peaks successively with continuous thermal tuning, and according to the thermal tuning coefficient and the self-injection locked range, the temperature control precision of better than 0.1 °C could assure the stable self-injection lock. On the whole, the simultaneous controlling of the temperature and drive current provides over 0.8 nm quasi-continuous tunability.
A self-delayed homodyne (SDH) beat note measurement is established to determine the laser linewidth. A 20-km-long delay fiber and an acousto-optic modulator with a frequency shift of 80 MHz are used in the measurement. The beat note signal is detected by an amplified PIN photodiode receiver module (PD1, Optilab, LR-12-A-M, bandwidth of 12 GHz) and then sent into a spectrum analyzer (SA, KEYSIGHT, N9030, 3 Hz~13.6 GHz). The beat note signal power spectra of the DFB diode laser with and without resonant optical feedback are shown in Fig. 3. In this experiment, the DFB diode laser is operated with a current of 131 mA at 34.36 °C. The resonant optical feedback injection ratio is adjusted to the best point, which leads to the narrowest linewidth. As shown by the gray curve in Fig. 3, the spectrum of the free-running DFB diode laser is broad and the linewidth obtained by a Lorentzian fitting is ~1 MHz. A significant reduction of the laser linewidth is achieved using resonant optical feedback (blue curve in Fig. 3), and an enlarged view of the spectrum with resonant optical feedback is shown in the inset of Fig. 3. A modulation pattern with a periodicity equal to the reciprocal delay time (the 20-km-long delayed fiber corresponding 10 kHz in our measurement) appears on the beat note spectrum, which means the coherence length of the laser exceeds the length of the delayed fiber, makes it difficult to estimate the linewidth.
The frequency-noise power spectral density (PSD) of the self-injection locked DFB diode laser is measured by an unbalanced Michelson interferometer composed of a 3 × 3 optical fiber coupler described in . The comparison results with and without resonant optical feedback are shown in Fig. 4. The resonant optical feedback from the cavity shows significant noise suppression in the Fourier frequency range in all of the measurements (with a measurements cutoff frequency of 1 MHz). At low Fourier frequencies, the frequency noise of the diode lasers is mainly 1/f noise and it is reduced substantially with resonant optical feedback from the FBG-FP cavity . According to , the reduction efficiency of the laser frequency noise at the low Fourier frequency by self-injection locking is related to the Q-factor of the resonator, and the higher the quality factor of the resonator the higher is the noise suppression. At high Fourier frequencies, the frequency noise of the diode lasers is mainly white noise, and it is suppressed effectively using optical feedback from a 4 m long external ring cavity [15, 16]. The frequency noise PSD of the self-injection locked DFB diode laser is reduced by larger than 70 dB with the Fourier frequencies between 5 Hz and 1 kHz, and above the Fourier frequencies of 1 kHz, the white noise plateau is kept as low as 40 Hz2/Hz, easing the burden on active frequency noise stabilization at high frequencies.
The frequency noise PSD can also provide an approach to evaluate the linewidth of the self-injection locked DFB diode laser, and the linewidth calculated for different integral bandwidths are shown in the Fig. 4. If we integrate above 1 kHz for 1 ms, FWHM linewidths of 600 Hz is obtained, which has been proven consistent with the Gaussian linewidths obtained using the SDH method [19, 21]. The white frequency noise level of S0 ~40 Hz2/Hz corresponds to an intrinsic Lorentzian linewidth of Δν = S0π ~125 Hz. The white frequency noise is supposed to be further improved by the FBG-FP cavity written on PM single mode fibers, which eliminates the distortion of the resonance transmission peaks caused by polarization mode coupling in a single-mode fiber. Moreover, a comparison of the frequency noise PSD for the self-injection locked DFB diode laser, RIO ORIONTM ECDL module, and Koheras BASIK fiber laser is shown in Fig. 4. The frequency noise PSD for the lasers are measured using the same setup. The self-injection locked DFB diode laser exhibits smaller frequency noise above the Fourier frequencies of 1 kHz; however, its frequency noise level is higher than the other two laser modules below 1 kHz because the whole system is not under temperature control shield or any particular vibration and acoustics insulation protections [12, 13, 21]. Future enhancement of mechanical stability and isolation from air turbulences are promising to provide improvement of spectral stability at acoustic frequencies.
The typical relative intensity noise (RIN) is measured using an InGaAs photodiode (PD) with a bandwidth of 1.2 GHz (Thorlabs, DET01CFC) and an electrical spectrum analyzer (KEYSIGHT, N9030, 3 Hz~13.6 GHz), and the laser power launched into the PD is kept at 3 mW in the entire measurement. Figure 5 shows the RIN of the free-running DFB diode laser (blue line) and the self-injection locked DFB diode laser with resonant optical feedback (red line). The measurement background noise limitation (gray line) is also shown for comparison, and the inset shows the low frequencies (<1 MHz) RIN. The RIN of the self-injection locked DFB diode laser presents a 1/f behavior up to about 2 MHz and remains flat at a level close to –142 dBc/Hz from 2 MHz up to 50 MHz, which exhibits the same characteristics as the free-running DFB diode laser. The noise floor of the RIN approaches the shot noise limit with the measured optical power of 3 mW at high frequencies, the actual RIN is probably even lower. It is noted that the noise spikes at low frequencies are attributed to the PD.
The power and the wavelength (optical frequency) variations of the self-injection locked DFB diode laser versus its swept driven current are important to the self-injection locked laser source, and these characteristics are plotted in Fig. 6(b), in which the characteristics of the original free-running DFB diode laser are also shown in Fig. 6(a) for comparison. The drive current, laser output power, and wavelength are monitored simultaneously by an oscilloscope and a wavelength meter at 34.36 °C, and the driven current is scanning from 103~137 mA, which corresponds to the gray area indicated in Fig. 2. Compared with the continuously varied optical power and wavelength of the free-running DFB diode laser, the self-injection locked DFB diode laser exhibits sudden changes in the optical power and wavelength variation curves, which is attributed to the mode hops by self-injection locking to the subsequent resonance transmission peaks during the drive current sweeping, and the wavelength step change of 0.036 nm equals to the FSR of the FBG-FP cavity. The inset of Fig. 6(b) is an enlarged view of the optical frequency tuning in the dashed circle, which corresponds to the DFB diode laser self-injection locked to one specific resonance transmission peak of the FBG-FP cavity near 1549.55 nm. The right axis represents optical frequency difference from 1549.551 nm, it can be mode-hop free tuned by 14 MHz near the specific resonance transmission peak.
In our experimental setup, the whole system is not under temperature control shield or any particular vibration and acoustics insulation protections yet. The passive stability of the self-injection locked DFB diode laser is restricted by the mode-hops between different orders of the FSR of long external ring cavity. Mode-hops are limited in great part by the relative drift of the external cavity longitudinal mode and the resonance transmission peak of the FBG-FP cavity. Since the external cavity is all-fiber configuration, the dominant contribution of the mode-hop is changes in the index of refraction of the fiber and the transmission property of the FBG-FP cavity resulting from the environmental temperature fluctuations. We measured the long-term wavelength stability of the laser at 34.36 °C and drive current of 117 mA over 120 min (see Fig. 7). The DFB laser can be self-injection locked to a single resonance transmission peak within frequency fluctuations of 660 MHz in 110 min, and then mode hop happens caused by longitudinal mode competition of the external ring cavity, brings out a sudden frequency change of ~25 MHz (FSR of the external fiber ring cavity). In the future work, the entire laser will be housed in an aluminum box enclosed in a polyurethane foam box for thermal and acoustic isolation, and an extra active electrical compensation for the drive current or temperature of the self-injection locked DFB laser operation will be introduced, we believe that a better long-term stability will be obtained.
In this paper, we present a subkilohertz laser system based on a commercial DFB diode laser self-injection locked to the resonant optical feedback of a FBG-FP cavity. The laser combines the merits of a long external cavity and high Q-factor FP cavity resonant optical feedback, suppressing the phase/frequency noise to the greatest extent. The frequency noise of the laser is as low as 40 Hz2/Hz above 1 kHz, and the performance of the laser is comparable to that of the Koheras BASIK fiber laser, while featuring low-intensity noise and convenient fabrication. The noise could be further reduced by the enhancement of mechanical stability and isolation from air turbulences. The proposed scheme has potential applications in interferometric fiber optic sensing, coherent optical spectrum analyzer, and phase-coherent laser communication and other advanced techniques that require a narrow linewidth.
This work is supported by the National Natural Science Foundation of China (NSFC) (61405218, 61535014) and Shanghai Natural Science Foundation (14ZR1445100).
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