We demonstrated a bi-directional, Er-doped dual comb fiber laser consisting of all-polarization-maintaining fiber devices. Polyimide films in which single-wall carbon nanotubes (SWNTs) were dispersed were used as the in-line saturable absorber. In order to avoid synchronization of the two combs and associated damage to the SWNT film, a two-branch configuration with two SWNT films was employed. Soliton pulses with almost the same optical spectra were generated stably in each direction, and dual comb beats were observed simply by overlapping the two outputs. The repetition frequency was 28 MHz, and the frequency difference was 105–140 Hz. Thanks to the small frequency difference, dual comb beats corresponding to the whole optical spectrum were observed without any overlapping. Fourier transform spectroscopy using the developed dual comb source was examined, and the characteristics of an optical filter were successfully obtained.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
An optical frequency comb is a light source that has longitudinal modes spanning with extremely high accuracy. The development of optical frequency combs resulted in breakthroughs in metrology applications, such as high-resolution spectroscopy, length measurement, and astronomy [1–3].
Among the applications of optical frequency combs, in high-speed spectroscopy, the dual-comb technique is very attractive [4,5]. However, the dual-comb technique needs two stabilized and synchronized comb systems, which makes the system large and complicated.
Recently, dual-comb lasers that generate two combs as the outputs have been reported by several groups using different approaches [6–8]. In dual-comb lasers, since two combs share the same laser cavity, they suffer the same perturbations. As a result, variations of the repetition frequencies, frep, are cancelled and variation of the frequency difference, Δf, can be suppressed. Among the known dual-comb systems, fiber-laser-type dual-comb sources show stable performance, and they show promise as practical candidates for simple dual-comb source. So far, several types of dual-comb fiber lasers have been demonstrated [6,9–16]. One of them is the “two wavelength oscillation type”, in which two different wavelength components oscillate simultaneously in one cavity [9,12,14,15]. In this dual comb, it is necessary to broaden the spectrum to overlap the two combs and observe the dual-comb beat signals.
A second type is the “bi-directional oscillation type”, in which two counter-propagating pulses oscillate in one fiber laser [6,10]. In this case, it is possible to achieve two counter-propagating combs at the same wavelength.
In practical applications, the stability, overlapping of the optical spectra, and the difference in repetition frequencies are important factors. The use of a polarization-maintaining (PM) fiber device is effective in improving the long-term stability and repeatability of fiber lasers. So far, all-PM type fiber lasers have been demonstrated by several groups [17–19]. Among the dual-comb fiber lasers, there have been three reports of PM ones [20–22]. They used a nonlinear fiber amplifying loop mirror or single wall carbon nanotubes (SWNTs) as the mode-locker. In those lasers, the oscillation wavelengths of the two combs were different, and there were some free-space components inside the fiber laser cavity. So far, an all-PM, all-fiber dual comb laser has not been developed.
In this work, we demonstrated an all-PM fiber type, Er-doped dual-comb fiber laser. A polyimide film in which single-wall carbon nanotubes (SWNTs) were dispersed was used as the mode-locker. In order to avoid automatic synchronization of the combs and damage to the film, we proposed a two-branch system. Bi-directional dual-comb mode-locking was achieved stably. The output spectra of the two combs were almost identical, and the dual-comb beat signals were observed simply by overlapping the two outputs. Thanks to the small frequency difference, the dual-comb beats of the whole spectra of the output pulses were observed without overlapping. The characteristics of the dual-comb output were investigated experimentally. In order to examine the applicability of the developed dual-comb source, Fourier transform spectroscopy of a wavelength filter was demonstrated.
Figure 1 shows the experimental setup of the all-PM Er-doped dual-comb fiber laser. A PM Er-doped fiber (EDF) was used as the gain device. The length was 1.2 m and it had normal dispersion properties. The PM-EDF was pumped by a high-power laser diode at λ = 1.48 μm. A 50:50 coupler was used as the output coupler. A polyimide film in which single walled carbon nanotubes (SWNTs) were dispersed was used as the saturable absorber. The SWNTs were synthesized by the HiPCO method and showed a wide absorption peak covering the 1.55 μm region . This film type device is easily inserted between fiber connectors, and it works as a transmission-type saturable absorber, which is useful for all-PM fiber lasers.
First, we examined a simple ring cavity configuration incorporating only one SWNT film, and we achieved bi-directional dual comb mode-locking operation . However, since the two beams irradiated one film, the total power was greater than the damage threshold of the film, which was ~5 mW, and dual-comb mode-locking could not be maintained for longer than a few tens of minutes in the all-PM configuration. In addition, when the frequency difference Δf was decreased, synchronization occurred between the two counter-propagating comb pulses, and the repetition frequency became the same. Therefore, we modified the cavity configuration as shown in Fig. 1. We used two optical circulators to separate the clockwise (cw) and counter-clockwise (ccw) pulses. In this scheme, the two pulses irradiated two different films, and the irradiation power could be kept below the damage threshold. Synchronization of the two combs did not occur in this scheme, even when the frequency difference was very small. Since one gain fiber was shared by two counter propagating pulses, it was important to control the optical loss in each direction to balance the gain for two pulses. So variable attenuators were used to control the optical loss inside the cavity to achieve dual comb mode-locking. They also played the role to reduce the irradiation power into SWNT film to avoid its damage. As a result, we successfully achieved stable, bi-directional dual-comb mode-locking operation over a long period of time.
Figure 2(a) shows the optical spectra of the output pulses. The net dispersion of the cavity was anomalous at −0.125 ps2, and sech2-shaped pulses with small Kelly sidebands were obtained stably by soliton mode-locking. The two optical spectra were almost identical, and the dual-comb beats were easily observed simply by overlapping the two outputs. The center wavelengths were 1562.6 nm and 1562.8 nm, and the spectral widths were 7.1 and 6.4 nm for cw and ccw pulses, respectively. The output power was 5.8 mW for the cw pulse and 1.2 mW for the ccw pulse. Although we did not use any polarizer inside the cavity, dual-comb mode-locking oscillation was achieved only along the slow axis. For the day to day operation, the dual comb mode-locking operation with almost the same optical spectra were always obtained after the adjustment of pump power. The dual comb mode-locking operation could be maintained more than 24 hours. Thanks to the all-PM configuration, we achieved high long-term stability and excellent repeatability.
Since the cw pulse came out just after the amplification in EDF, and the ccw pulse came out after passing through the variable attenuator and the SWNT film, the output power of cw pulse was larger than that of ccw one. It is considered that if we took the outputs from the two branch parts, especially after the SWNT films, we would be able to obtain almost the same output power for the two pulses.
Figure 2(b) shows the observed RF spectra of fundamental frequencies for two output pulses. In order to observe the dual-comb beats of the whole optical spectra, the relation N・Δf ≤ frep/2 must be satisfied, where N is the mode number, Δf is the difference of the repetition frequencies, and frep is the repetition frequency. We carefully adjusted the fiber lengths of the branch parts to satisfy the above relation. In Fig. 2(b), the repetition frequencies were 27,974,857 Hz and 27,974,973 Hz for the cw and ccw pulses, respectively. The frequency difference Δf was 116 Hz in this condition, and stable mode-locking was achieved. The corresponding length difference of branch parts was 40 μm. From the optical spectra shown in Fig. 2(a), the estimated mode number N was ~110,000, and the above relation was successfully satisfied. As described later, the magnitude of Δf could be controlled using the pump current control and PZT actuator.
Figures 2(c) and 2(d) show the observed RF spectra of the cw pulse output. From Fig. 2(c), almost flat RF beats of longitudinal modes were observed in a wideband region, and stable mode-locking was confirmed. In Fig. 2(d), a high SNR of 80 dB was observed when RBW = 10 Hz, confirming the low noise properties of the developed dual-comb fiber laser. Almost the same properties were observed for the ccw output.
Figure 2(e) shows the observed autocorrelation trace for the cw output. A background-free SHG-type autocorrelator (Femtochrome Research, FR-103XL) was used for the measurement. A pedestal-free, almost symmetric autocorrelation trace was observed. The temporal width was 982 fs, and the corresponding pulse width was estimated to be 636 fs under the assumption of a sech2 pulse. The transform-limited pulse width estimated from the spectral width of 7.1 nm was 361 fs. The large temporal broadening was caused by the chromatic dispersion of the output couplers and optical isolator. The small variation of the background noise level was due to the effect of the low SNR of this measurement owing to the low peak power of the broadened output pulses. For the ccw output, since the output power was 0.6 mW, the autocorrelation trace was not observed. The estimated transform-limited pulse width was 395 fs.
Figure 2(f) shows the observed pulse train of the output pulses. The cw and ccw outputs were overlapped with a fiber coupler, and the temporal pulse train was observed with a fast pin photodiode (Thorlabs DET01CFC) and a fast digital oscilloscope (Yokogawa DL9240L). Two overlapped pulse trains with slightly different repetition rates were observed. When one pulse train was synchronized, the other pulse train moved in the window with a constant speed. Using the digital oscilloscope, dual-comb operation was confirmed in the temporal domain.
If we increase the magnitude of the attenuation in the variable attenuator, we could achieve conventional single-pulse mode-locking operation for each direction. We examined the output properties for single-pulse operation, and almost the same properties as those of the dual-comb operation shown in Fig. 2 were confirmed.
Next, we examined the long term stability of the developed dual-comb fiber laser. Figure 3(a) shows the variations of two the repetition frequencies, frep, and frequency differences, Δf, for 100 minutes. The dual-comb fiber laser was covered with an acrylate box, and a water-cooled breadboard was used to stabilize the temperature inside the box. The maximum variation of frep was 7.7 Hz, and that of Δf was 0.78 Hz. When we examined this stability characteristics for 7 hours, the maximum variation of frep was 20 Hz, and that of Δf was 1 Hz. The two repetition frequencies, frep, varied in the similar manner, and we confirmed that the variation of Δf was suppressed by employing the dual-comb fiber laser scheme.
In this developed laser, stable dual-comb mode-locking operation was achieved for a wide range of pump LD currents. Figure 3(b) shows the variations of frep and Δf as a function of pump LD current. As the pump current was increased, the magnitudes of the repetition frequencies decreased. The center wavelengths of the mode-locked pulses were shifted towards the longer wavelength side as the pump LD current was increased. Therefore, it was considered that the shift of frep was caused by the wavelength shift of the mode-locked pulse and the anomalous dispersion properties of the fiber laser cavity.
Regarding Δf, the magnitude of Δf decreased as the pump power was increased. This was caused by the different behavior of the frep shift between the cw and ccw outputs with respect to changes in the pump LD current. The frequency difference Δf changed in almost a linear manner. This property will be useful for feedback control of Δf in future.
Next, we examined the dual-comb RF beats of the developed dual-comb fiber laser. The two outputs were overlapped with a 50:50 PM fiber coupler, and then the overlapped beams were introduced into a balanced detector (Thorlab PBD150C). The output from the detector was observed with a high-speed digital oscilloscope (Yokogawa DL9240L) and RF spectrum analyzer (Anritsu MS2830A).
Figure 4 shows the observed temporal interference signals of the dual-comb fiber laser. A low pass filter was used at the input of the digital oscilloscope. Sharp interference signals were observed periodically, and single-pulse operation was confirmed for both directions. In the magnified view of the graph, we can see the interference fringe pattern. Averaging was effective to increase the SNR for this measurement. The small interference signals before and after the main peak were observed clearly by averaging.
Figure 5 shows the observed RF spectra of the dual-comb fiber laser output. A PZT was used to adjust the position of the dual-comb beats to observe the beats of the whole comb without overlapping. Thanks to the small Δf, the RF dual-comb spectra corresponding to the whole optical pulse spectra were successfully observed without overlapping. We could see the spectral shape with the Kelly sideband components. The shape of the RF beats was well fitted with that of optical spectra. For the enlarged view shown in Fig. 5(b), comb modes with 116 Hz interval were clearly observed.
Next, we examined the feasibility of dual comb spectroscopy using the developed dual-comb fiber laser. In this work, we used narrow band wavelength filters as samples.
Figure 6 shows the observed temporal interference signals and obtained optical spectra. A wavelength-tunable band pass filter was used as the sample, and it was set in the output port of the ccw pulse. The temporal interference signal was observed in the digital oscilloscope, and the waveform after averaging 128 times was recorded as the observed results. Then, the FFT of the observed temporal waveform was calculated, and RF dual-comb beats were obtained. Theoretically, the minimum measurement speed is equal to 1/Δf = 8.6 ms . The averaging is effective to improve the SNR of the measurement.
When the filter was not used, a narrow, almost symmetrically shaped interference signal was observed. The obtained RF beat spectra showed clear spectral shapes, which were well fitted with the optical pulse spectra. When a wavelength-tunable bandpass filter with a bandwidth of 0.6 nm was used in the ccw output, the tail part of the interference signal was broadened. Using FFT analysis and deconvolution, the transmitted spectra were obtained clearly in the RF spectral domain. We set the center wavelength at three different wavelengths. The obtained spectra were well fitted to the observed optical spectra after unit calibration. We confirmed the feasibility of dual-comb spectroscopy for the developed dual-comb fiber laser. Phase information can be obtained in addition to intensity information by using additional optics and analysis . The wideband measurement can be demonstrated using the fiber amplifier and nonlinear spectral broadening process in future.
We demonstrated a bi-directional, all-polarization-maintaining, Er-doped dual-comb fiber laser for the first time. Single-wall carbon nanotubes (SWNTs) dispersed in a polyimide film were used as the saturable absorber for passive mode-locking. In order to avoid synchronization of the two combs and associated damage to the SWNT film, a two-branch configuration with two SWNT films was employed. Stable dual-comb operation by soliton mode-locking was achieved, and the optical spectra for the two outputs overlapped well. The repetition frequencies were ~28 MHz, and the frequency difference was varied from 105 to 140 Hz by changing the pump LD current. The variation of the frequency difference, Δf, was below 0.74 Hz, and cancellation of the variation in frep was observed. Thanks to the small Δf, dual-comb beats corresponding to the whole optical spectra were clearly observed without any overlapping with the mirror image. The developed dual-comb fiber laser was applied to Fourier transform spectroscopy, and clear transmitted spectra of a narrow bandpass filter were obtained, and these were in good agreement with the observed optical spectra.
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