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Abstract
An all-polarization-maintaining dual-comb fiber laser with a mechanical shared-cavity configuration was demonstrated. The laser cavity configuration was simplified and downsized using the micro-optic component of a saturable absorber mirror and a wavelength-division multiplexer. A high relative frequency stability was achieved with an Allan deviation of 0.02 Hz. Further, the all-polarization-maintained fiber-based configuration facilitated an integrated phase noise of the relative beat note between dual-frequency combs of 378 rad (10 Hz−1 kHz) and 9.0 rad (100 Hz−1 MHz). The simple, compact, and robust dual-comb fiber laser yielded highly mutually coherent dual-optical frequency combs without active servo control, and significantly simplified dual-comb spectroscopy.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical frequency combs are versatile tools used in various technical and scientific fields such as metrology, high-resolution spectroscopy, precision ranging, and astronomical calibration [1,2]. Dual-comb spectroscopy has garnered significant interest because of its advantages over other spectroscopic techniques in terms of data acquisition time, sensitivity, and resolution [3–7]. Owing to its excellent specifications, dual-comb spectroscopy has been used in several applications [8–11]. It is a type of Fourier transform spectroscopy wherein multi-heterodyne detection is performed using two optical frequency combs with slightly different repetition rates (freps). However, the two frequency combs must have high relative-frequency stability and mutual coherence to realize high-resolution, broadband, and high-sensitivity spectroscopy.
A conventional dual-comb spectroscopy system employs two independent mode-locked lasers to generate two optical frequency combs with slightly different frep (Δfrep). However, independent fluctuations in the free-running operation result in a relative frequency instability between the two frequency combs. Therefore, mutual coherence is reduced and the interferogram (IGM) is destructive. The tight phase-locking technique [6–9,12–14], high-speed signal processing with an Field Programmable Gate Array (FPGA) [15], or self-phase tracking technique [16] are typically used to suppress the relative frequency instability and obtain a high mutual coherence between the two frequency combs. Despite the superior specifications of dual-comb spectroscopy, it is mainly restricted to research related to laser frequency stabilization because of the complex servo system. Thus, a simple, compact, robust, and versatile dual-comb laser source could offer an alternative solution for a wide range of users, including non-experts.
Recently, a dual-comb laser capable of generating two optical frequency combs with a small Δfrep from a single laser cavity has attracted attention owing to its high relative-frequency stability and mutual coherence through common-mode noise suppression in free-running operations. This is because the two frequency combs share the same laser cavity [17–26]. Various dual-comb fiber lasers have been reported based on dual-wavelength [17], dual-polarization [18,19], and bidirectional [20–22] mode-locked fiber lasers, which are inexpensive, compact, and robust owing to their all-fiber based configuration. However, most dual-comb lasers require a specific laser cavity design. To overcome this problem, we demonstrated a mechanical sharing all-polarization-maintaining (PM) fiber-based dual-comb laser [27] to simplify cavity design and obtain high relative-frequency stability between the two frequency combs with a nonlinear amplifying loop mirror (NALM) mode-locking mechanism [28,29]. A mechanical sharing cavity configuration has also been demonstrated in a solid-state laser [30]. An all-PM mode-locked fiber laser is robust in practical environments [31], and an all-PM mode-locked fiber laser with a NALM mode-locking mechanism exhibits low phase noise capability for optical frequency comb generation [28,29] compared to PM fiber lasers with a saturable absorber mirror [32]. Owing to the common-mode noise suppression, a low integrated phase noise of 420 rad was obtained, which was estimated through the integration of the spectral density of the phase noise from 10 Hz to 1 kHz [27]. However, the NALM mode-locking mechanism requires a complex optical component, that is, a nonreciprocal phase shifter in free space. Therefore, the mechanically shared dual-comb fiber laser could not be simplified and miniaturized.
This study developed an all-PM and compact dual-comb fiber laser based on two independent all-PM fiber lasers that mechanically shared laser cavities using a micro-optic component. The components included a semiconductor saturable absorber mirror (SESAM), focusing optics, and a wavelength-division multiplexer (WDM). The SESAM mode-locking mechanism is disadvantageous in terms of phase noise compared to NALM; however, it can be used with a simple laser cavity configuration, that is, a micro-optic component. The two frequency combs, generated from independent cavities, exhibited high relative frequency stability owing to the implementation of technical common-mode noise suppression with a mechanical sharing laser cavity. Moreover, the absence of a free-space section allowed the dual-comb fiber lasers to be compact and robust. Consequently, we realized high relative frequency stability with an Allan deviation of 0.02 Hz. Furthermore, an integrated phase noise of the relative beat note between the two frequency combs of 378 rad (10 Hz−1 kHz) and 9.0 rad (100 Hz−1 MHz) was realized owing to the all-PM fiber-based configuration.
2. Experimental setup
Figure 1 illustrates a schematic of the all-PM dual-comb fiber laser, comprising two all-PM mode-locked fiber lasers with a micro-optic component (AFR, PMFSESAM-55-1-B-Q-33-F-P) containing a SESAM (BATOP, SAM-1550-33-2pc-1.3b-0), focusing optics, and a WDM coupler. The laser comprised a linear cavity bound by the SESAM, a PM-Er-doped fiber (EDF, Liekki, Er-80-4/125/HD-PMF) spliced to a PM non-doped fiber (PMF), and a partial reflected mirror (PRM) with an 80% reflective dielectric mirror. The PM-EDF was coupled into a micro-optic component containing a SESAM and pumped by a 976-nm laser diode (LD) via a WDM coupler. As shown in Fig. 1, all the components in the two-laser cavity were placed at the same position in a diecast aluminum box (450 mm × 300 mm × 120 mm) housing for common-mode environmental disturbances. Therefore, a two-laser cavity mechanically shared the technical noise. Such a simple laser cavity comprising a micro-optic component, PM-EDF, and a PRM simplified the installation of laser cavities at the same position. Moreover, to improve the common-mode noise suppression, the PM-EDFs in the two laser cavities were pumped by the same LD via a 50/50 optical coupler (CPL) operating at 980 nm. In this study, all the experiments were conducted in a free-running operation, for example, no use of temperature control, as described in the following section.
Fig. 1. Schematic of an all-PM dual-comb fiber laser with mechanical sharing cavity and micro-optic package. LD: laser diode; CPL: Coupler; SESAM: semiconductor saturable absorber mirror; EDF: Er-doped fiber; PMF: polarization-maintaining fiber; PRM: partial reflected mirror; ISO: isolator. (Inset) shows a picture depicting the placement of the lasers.
3. Experimental results
3.1 Characteristics of both laser outputs
Both laser outputs (comb1 and comb2) were measured using an optical spectrum analyzer (Yokogawa, AQ6370D) and a radio-frequency (RF) spectrum analyzer (RIGOL, DSA815) via a fast photodetector (Newfocus, 1611). With increase in the pump power, the mode-locking operation was self-initiated owing to the SESAM. Whereas, as the pump power decreased, dual-comb operation was realized. Figure 2(a) shows the optical spectra of the two mode-locked fiber laser outputs [comb1 (red) and comb2 (blue)]. In the comb 1 output, the center wavelength, full-width at half-maximum (FWHM) bandwidth (Δλ) of the spectrum, and output power were 1558.4 nm, 7.1 nm, and ∼1 mW, respectively. Whereas for comb 2 output they were 1559.5 nm, 7.1 nm, and ∼1 mW, respectively. The slight difference in the optical spectra between the two outputs was owing to the slight difference in the length of the PM-EDF. Figure 2(b) shows the RF spectra of combs 1 and 2 at a resolution bandwidth (RBW) of 300 kHz. The frep for the two frequency combs was 48.872 MHz, and Δfrep was 182 Hz.
Fig. 2. (a) Optical spectra and (b) (left) RF spectra of two laser outputs, (right) zoomed in view of the first harmonics. (c) Temporal variation in the repetition rates of the two frequency combs and (d) the difference in the repetition rate.
To evaluate the relative-frequency stability in the long term, we measured frep for the two frequency counters with reference to the Rb frequency standard (SRS, FS725) at a gate time of 1 s. Figure 2(c) shows the temporal variation in frep for the two frequency combs during the free-running operation. A change of approximately 120 Hz was observed in frep owing to the environmental perturbations. In contrast, as shown in Fig. 2(d), a high relative-stability was obtained for Δfrep with a standard deviation of 0.71 Hz (Allan deviation of 0.02 Hz at average time of 1 s) at Δfrep of 180 Hz, owing to the mechanical sharing laser cavity configuration. The achievement of such a small Δfrep in a single-laser cavity dual-comb fiber laser has not been previous reported, primarily owing to the cross-talk between the two frequency combs [19]. As shown in Fig. 2(d), Δfrep has a residual drift component owing to the non-common-mode noise. This can be compensated for by controlling the temperature of the sharing laser cavities.
3.2 Relative-frequency stability evaluation in short term
To evaluate the relative frequency stability between the two frequency combs of the two lasers in the short term, we detected the beat notes (fbeat1, fbeat2) between each frequency comb and a narrow-linewidth single-frequency laser (RIO, PLANEX) at 1566 nm with a high signal-to-noise-ratio (SNR) of approximately 30 dB at an RBW of 300 kHz [Fig. 3(a), (b)]. The phases of the two signals were measured using a multichannel digital phase meter [33], simultaneously. The phase noises of the two fbeat signals and the difference Δfbeat yielded a common-mode noise rejection ratio of 50 dB at a frequency of approximately 1 Hz [Fig. 3(c)]. The phase noise of Δfbeat, that is, the relative-frequency stability in the short term, indicated that the phase noise of the single-frequency laser and both comb line modes were successfully canceled out. Further, the measurement limit was determined according to the SNR of fbeat detection. In addition, a wide-dynamic range characterization (−100−140 dB in 0.1 Hz −10 MHz) was achieved using the digital phase meter. Based on the difference indicated in Fig. 3(c), the analysis employing the β-separation line [34] yielded a relative linewidth of 18 kHz. The integrated phase noise (100 Hz−1 MHz) of the two beat notes and Δfbeat were 102, 101, and 9.0 rad, respectively, as shown in Fig. 3(d). In particular, the integrated phase noise of Δfbeat of 378 rad (10 Hz−1 kHz) was less than that reported in previous research with a NALM mode-locking mechanism (420 rad) [27]. This improvement can be primarily attributed to the incorporation of the micro-optic component, rather than a difference in the mode-locking mechanism. Figure 3(e) shows the temporal variations in the two beat notes and Δfbeat. A temporal perturbation in the two beat notes was canceled out in Δfbeat owing to the common-mode noise cancellation. Figure 3(f) shows the Allan deviation of the two beat notes (fbeat1: red, fbeat2: blue) and Δfbeat (black). A drastic improvement in relative stability was observed. The remarkably low values of the relative frequency stability incorporated the difference in the carrier-envelope offset frequency between the two frequency combs. However, we believe that the highly stable relative-frequency stability can be attributed to the all-PM fiber laser configuration, which does not include free space.
Fig. 3. Beat notes between a single frequency laser and the two laser outputs in comb1 (a) and comb2 (b) at 1561 nm with an RBW of 300 kHz. (c) Phase noise and (d) integrated values of the beat notes (comb1, comb2) and the difference (diff). (e) Temporal variation in the beat notes. (f) Allan deviation of the two beat notes (fbeat1, fbeat2) and Δfbeat (black).
3.3 Spectroscopy with a developed dual-comb fiber laser
Finally, we performed spectroscopy using the developed mechanical-sharing dual-comb fiber laser. These two outputs were launched into two homemade PM-EDF amplifiers for spectral broadening. Subsequently, the two outputs were launched into a fast photodetector (Newfocus, 1811) via an HCN cell (Wavelength References, HCN-13-H(5.5)-25) and an optical bandpass filter with a bandwidth of ∼9.2 nm at 1551 nm. The temporal IGM was observed using a digital oscilloscope (Tektronix, MSO44). Figure 4(a) shows the dual-comb IGM acquired via the detection of two spatially overlapped frequency combs with Δfrep of 140 Hz. Figure 4(b) shows a magnified IGM of that shown in Fig. 4(a) with approximately 10 µs around the center burst point. Thereafter, dual-comb absorption spectroscopy was performed on the HCN cell with a spectral resolution of 2.0 GHz (15 pm) and an acquisition duration of approximately 200 µs, centered at 1551 nm. As shown in Fig. 4(c), the spectrum demonstrated the absorption line of HCN. On the other hand, in a fully free-running dual-comb fiber laser, the frep exhibits significant fluctuations, as depicted in Fig. 2(c). Furthermore, the instability of the developed laser complicates the integration of the interferogram. This is because the gain medium is PM-EDF with normal dispersion. The spectrum was consistent with that of the same HCN cell obtained using an optical spectrum analyzer [Fig. 4(d)]. A significant long sweep time of approximately 5 s was required to realize a spectral resolution of 6.2 GHz (50 pm). Instead, a higher signal-to-noise ratio was obtained compared to the dual-comb fiber laser.
Fig. 4. Spectroscopy of an HCN cell with a mechanical sharing dual-comb fiber laser (Δfrep = 140 Hz). (a) Temporal interferogram, (b) magnified signal of (a), and (c) dual-comb spectra obtained through the implementation of Fourier transform on the interferogram (a) with acquisition duration of 20 µs around the burst point. (d) Transmitted spectrum of the same cell obtained by using an optical spectrum analyzer.
4. Conclusions
This study developed an all-PM, compact dual-comb fiber laser based on two independent all-PM fiber lasers that mechanically shared laser cavities using a micro-optic component of a SESAM and a WDM. A micro-optic component was suitable for compact setup and low relative phase noise operation. The absence of a free-space section ensured compact and robust the formation of dual-comb fiber lasers. Consequently, we realized high relative frequency stability with an Allan deviation of 0.02 Hz. The integrated phase noise of the relative beat note between the dual-frequency combs of 378 rad (10 Hz−1 kHz) and 9.0 rad (100 Hz−1 MHz) was realized owing to the all-fiber-based configuration. In a mechanical sharing configuration, unnecessary nonlinear interactions can be prevented between the two frequency combs. Here, we demonstrate the spectroscopy of HCN gas with extra-cavity nonlinear spectral broadening for a strong HCN gas absorption line with a bandwidth of ∼9.2 nm. The all-PM-based compact configuration was robust against practical environmental perturbations. In addition, the long-term relative stability could be improved even without a complicated tight phase-lock. Moreover, the use of digital phase correction [16,,35,36] can ensure further improvement in frequency stability. A practical dual-comb spectroscopy system based on a mechanical sharing all-PM dual-comb laser can be used beyond special laboratories and without expertise in laser stabilization. Furthermore, the flexible mechanical sharing laser configuration is applicable to a broad range of platforms beyond dual-comb spectroscopy, such as synchronization or asynchronization between ultrashort pulse trains with different colors for various nonlinear optical measurements, and integrated fiber sensing systems with optical sources and sensing parts.
Funding
Adaptable and Seamless Technology Transfer Program through Target-Driven R and D (JPMJTM22B6); Research Foundation for Opto-Science and Technology; Hattori Hokokai Foundation; Mayekawa Houonkai Foundation; Japan Society for the Promotion of Science (22H00303).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this letter are not publicly available at this time but may be obtained from the authors upon reasonable request.
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