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Abstract
We propose and demonstrate a novel dynamically tunable fiber-based Lyot filter for the realization of a dual-wavelength mode-locked fiber laser, operating at center wavelengths of 1535 nm and 1564 nm. The same laser cavity can also be operated in a single-wavelength mode-locked regime with a wavelength tuning range of 30 nm, from 1532 nm to 1562 nm. The proposed dynamically tunable Lyot-filter provides a simple setup for laser mode-locking using a single laser cavity design to generate dual-wavelength pulses, with the flexibility to also allow the generation of single-wavelength pulses with a continuously-tunable center wavelength.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tunable optical comb filters such as fiber Bragg gratings (FBG) and Lyot filters [1,2] have been deployed to realize multi-wavelength laser operation, for wavelength-division-multiplexing (WDM) optical communications and various other applications. The Lyot filter has the advantage of a broad continuous wavelength-dependent transmission spectral characteristics, compared to FBG, which suffers from a limited bandwidth and a relatively large group-delay ripples. However, in most of the previously demonstrated comb filters, the channel spacing of the filter transmission spectrum is fixed and not adjustable.
The Lyot filter which was invented by Bernard Lyot in 1933 is a type of comb filter with a broadband operation characteristic. It has been used in various applications such as spectral imaging [3,4], optical communications [5] and laser systems [6–9]. A fiber-based Lyot filter is commonly constructed by placing a section of birefringent fiber, such as a length of polarization maintaining fiber (PMF), in between two polarizers [10–13]. The wavelength spacing of the filter can be tuned by changing the length of the PMF. A discrete step-wise tuning of the effective fiber length has been demonstrated by rotating two segments of PMF [14]. However, this is not a practical technique for dynamic and continuous tuning of the filter wavelength spacing. A small but significant amount of birefringence induced by the twisting and squeezing of the fiber in the loop-type polarization controller (PC) is sufficient to affect the spectral property of the Lyot filter. Additionally, the stress-induced birefringence will also be affected by the rotation of the PC during polarization tuning operations. The piece of optical fiber installed within the loop-type PC would act as an optical element with a tunable birefringence, so that the total birefringence of the Lyot filter can be tuned [15]. Therefore, a very simple continuously tunable fiber comb filter can be obtained by making use of the birefringence derived from the PC.
Recently, there is an increasing research interest in dual-wavelength mode-locked fiber lasers, capable of generating pulses at two different wavelengths from a single laser cavity, for applications such as dual-comb spectroscopy and terahertz wave generation. In order to realize dual-wavelength mode-locking in a fiber laser, a programmable attenuator can be inserted into laser cavity to control the spectral gain profile of the erbium doped fiber (EDF) to balance the gain competition between the two lasing wavelengths [16]. However, it is very difficult to stably control the stringent gain condition of the EDF by changing only the cavity loss. New methods with better flexibility and stability are needed for practical applications. On the other hand, wavelength tunability in a mode-locked laser is also an important feature required for applications such as spectroscopy, optical communications, fiber-optics sensor and biomedical research [17–19]. Various devices have been used to tune the wavelength of a mode-locked laser, such as thin-film-based optical band-pass filters (OBPF) and FBG [20,21]. However, these devices suffer from a limited wavelength tuning range and a relatively high insertion loss.
In this paper, we propose and demonstrate a mode-locking fiber laser capable of operation in a dual-wavelength regime as well as a single-wavelength regime with wavelength tunability over the filter’s free spectrum range (FSR) of 30 nm. The key element is a fiber-based Lyot filter which can be tuned via simple adjustment of a PC. The central wavelengths of laser output at the dual-wavelength regime are 1535 nm and 1564 nm. The wavelength tuning range at the single-wavelength regime is from 1532 to 1562 nm, which covers the whole of the C-band. The wavelength tuning range can be extended to S-band or L-band by choosing different type of EDF such as a depressed cladding EDF or using longer length of EDF.
2. Experiment setup
The transmission characteristics of a Lyot filter can be written using Jones Matrix function as [15]:
Here, we design a dynamically tunable Lyot filter with an FSR of 30 nm, using a 23-cm length of PMF, chosen according the above equations. The schematics structure of this filter is shown in Fig. 1. As illustrated above, the PC with bent and twisted fiber inside is sufficient to affect the transmission of Lyot filter. The calculated transmission spectral characteristics of the designed Lyot filter are shown in Fig. 2(a) with varying the total birefringence and Fig. 2(b) with varying polarization angles. In Fig. 2(a), the different colors refer to different total birefringence, while the polarization angle is kept unchanged. The change in the stress-induced birefringence is estimated to be about 10−7, which is a thousand times smaller than the birefringence of the 23-cm length of PMF. Although this change of birefringence can only change the FSR slightly, since the frequency of the 1550 nm is near 192 THz, the transmission near 1550 nm will experience a lot of rounds of period. Therefore, this small change will definitely influence the central peaks of the Lyot filter. Here, in this simulation, the change of the total birefringence is about 1/1000 scale of the PMF according to the stress-induced birefringence. From the simulation result, it is shown that the position of the spectral transmission peak can be tuned by a slight change in the total cavity birefringence, which is consistent with our theoretical analysis. So, this spectral transmission property with a broad FSR is suitable for wavelength tuning of mode-locking operation. In Fig. 2(b), the different colors refer to the different polarization angles of the PC, while the total birefringence is kept constant. This figure shows that by changing the polarization angle of the PC, the spectral transmission amplitudes can be adjusted, which is consistent with Eq. (1). Because in Eq. (1), (1+$\textrm{sin}2\theta $) represent the amplitude function of this transmission profile, which is depending on the $\theta $. Such effect can be used to balance the two prominent gain peaks in the EDF gain spectrum. Therefore, a dual-wavelength mode-locking operation can be achieved with this filter effect. As mentioned above, by rotating the PC, both polarization angles and the total birefringence will be changed. So, with this dynamically tunable Lyot filter, the transmission profile should be the combination of both Fig. 2(a) and Fig. 2(b). Based on the above analysis, the peak position and amplitude of the transmission spectrum can be tuned by simple adjustment of the PC.
Fig. 2. Transmission spectra of the Lyot filter with different colors corresponding to: (a) total birefringence (simulation), (b) polarization angle (simulation), and (d) state of the PC (experimental), (c) test system of tunable Lyot filter’s transmission.
We have carried out an experiment which is shown in Fig. 2(c) to test the spectral transmission property of the designed Lyot filter: The amplified-spontaneous emission (ASE) from a commercial polarization-maintaining erbium-doped fiber amplifier (EDFA) (Amonics AEDFA-PM-23-B-FA) is used as the test source and a 50/50 optical coupler (OC) is utilized to split the source into two paths: one measured directly using an optical spectrum analyzer (OSA) as the reference level, while the other path is launched through the tunable Lyot filter before measurement using the OSA. The measured spectral transmission of the tunable Lyot filter is shown in Fig. 2(d), where different colors of the plots refer to different states of the PC. The traces show different spectral peak positions and amplitudes under different rotations state of the PC, which agrees well with the above simulation results.
The schematics of the laser experiment setup is shown in Fig. 3. A 5 m-long EDF is used as the gain media with an estimated normal dispersion of ∼50 ps2/km at 1550 nm. The EDF is pumped using a 980nm pump laser diode, through a 980/1550 wavelength-division multiplexing (WDM) coupler in a forward-pumping scheme. An optical isolator (ISO) is used to ensure unidirectional operation and the laser is mode-locked using a carbon nanotube (CNT) saturable absorber [23] comprised of a sandwiching of fiber connectors spray-coated with CNT with an insertion loss of 1.2 dB. The tunable Lyot filter is inserted in the laser cavity to serve as a wavelength selecting element. The 20% output from the 20/80 optical coupler (OC) is used as the output port. An additional 13 m length of standard single-mode fiber (SMF) with an estimated anomalous dispersion of -17 ps2/km at 1550 nm is inserted between the WDM and the OC for dispersion management. The pigtails of all the passive fiber components are made of standard SMF and the total cavity length is approximately 30 m. The optical spectrum, temporal waveform, radio frequency (RF) spectrum and the autocorrelation trace of the laser output are measured using an OSA (YOKOGAWA 6375), a 4 GHz oscilloscope (KEYSIGHT DSO-S 404A), a RF spectrum analyzer (RIGOL DSA832) with a 5 GHz photo-detector (PD, DET08CFC/M) and an autocorrelator (Femtochrome FR-103 XL) with a fiber-pigtailed input, respectively.
Fig. 3. Schematics of the mode-locked ring-cavity laser with CNT saturable absorber and a tunable Lyot filter. (WDM: wavelength-division multiplexer; EDF: erbium-doped fiber; PI-ISO: polarization-insensitive isolator; PMF: polarization maintaining fiber; PC: polarization controller; OC: optical coupler; SMF: single-mode fiber).
3. Results and discussions
3.1 Dual-wavelength mode-locking output
The gain spectrum of the EDF gain spectrum has two prominent gain peaks at 1532nm and 1555nm with gain levels dependent on the pumping and gain saturation condition. However, due to the mode competition between the two gain peaks, typically, only single-wavelength mode-locked pulses can be obtained when the pumping and gain saturation condition of the laser cavity is not carefully matched. In the proposed laser, the mode competition can be suppressed easily by simple adjustment of the PC to tune the transmission spectral profile to allow dual-wavelength mode-locking operation. The threshold power of dual-wavelength mode-locking operation is ∼17.2 mW. The output center wavelengths of the laser are at 1535 and 1564 nm, as shown in Fig. 4(a). The laser changes to multi-pulsing operation at 1564 nm when the pump is above 40.1 mW, with the total output power of ∼489.6 μW under 40.1 mW pump power. The temporal waveform of the pulse train operating in the dual-wavelength mode-locking regime is depicted in Fig. 4(b). The measured pulse period of ∼ 150 ns is consistent with the total cavity length of ∼ 30 m. The RF spectrum is shown in Fig. 4(c) with a repetition rate of 6.38 MHz. By measuring the pulses in RF domain with a smaller resolution bandwidth (RBW) of 10 Hz, different repetition rates of these two-color pulses can be observed. The difference in repetition rate is measured to be ∼340 Hz, which is due to the group velocity dispersion (GVD) in the cavity resulting in different round-trip group delay at the two separated lasing wavelengths. The repetition rate difference can be tuned by using different fiber types to provide different average GVD in the laser cavity. The signal to noise ratio (SNR) of these two pulses are 51 dB and 53 dB, as measured on the RF spectrum in Fig. 4(d).
Fig. 4. (a) Output characteristics of the dual-wavelength mode-locking fiber laser: (a) Optical spectra (resolution: 0.05 nm), (b) Temporal oscilloscope waveform, (c) RF spectrum (resolution bandwidth: 1 kHz), and (d) RF spectrum (resolution bandwidth: 10 Hz).
The optical spectra and autocorrelation traces of the pulses at 1535 nm and 1564 nm can be measured separately by using a bandwidth- and wavelength-tunable flat-top OBPF (Alnair Labs BVF-200) to filter out each wavelength component before measurement. As shown in the Figs. 4(a) and 5(a), the 1535 nm output pulse component has a spectral half-width of 4 nm and an inferred full-width at half-maximum (FWHM) pulse-width of 853 fs assuming a Gaussian waveform. Also, as shown in the Fig. 4(a) and 5(b), the spectral half-width and the inferred FWHM pulse-width of the 1564 nm pulse component is 3.6 nm and 1000 fs, respectively. The time-bandwidth products (TBPs) for the 1535 nm and 1564 nm pulses are 0.442 and 0.441, respectively, which are near to the transform-limited value of 0.441 assuming Gaussian waveforms.
3.2 Wavelength tunable mode-locking output
The mode-locking fiber laser can be operated in a single-wavelength mode-locked regime, by tuning the PC, with center wavelength ranging from 1532 nm to 1562 nm, as shown in Fig. 6. Because of the GVD in the laser cavity, the mode-locked pulses at different wavelength is expected to operate in slightly different soliton conditions, as can be noticed from the changing spectral shapes and Kelly sidebands at different operating center wavelengths. Additionally, since the amplitude of the spectral transmission peaks vary for the different PC states, this results in the variation in output intensity of the pulses when tuned to operate at different wavelengths. It also shows a 30 nm wavelength tunable range which covers the whole C-band gain spectrum. Since the Lyot filter has a continuous wavelength-dependent transmission band in the spectra, the same filter can potentially be applied to other wavelength regimes using gain media such as thulium-doped fiber and ytterbium-doped fiber.
Fig. 6. Output optical spectra in single-wavelength mode-locking regime (center wavelength tuned from 1532 nm to 1562 nm.).
In order to further confirm the functionality of the tunable Lyot filter, the PMF and polarizer are removed from the cavity on purpose. In this case, only single-wavelength mode-locking output at 1556 nm can be generated. This truly indicates that the proposed tunable Lyot filter is the key device to enable the operation of the dual-wavelength mode-locking regime, and the tunable single-wavelength mode-locking regime. In addition, the stability of the dual-wavelength mode-locking fiber laser is tested over 24 hours without any degradation in laser output performance.
4. Conclusions
In conclusion, we have proposed and demonstrated a novel fiber laser design using a fiber-based tunable Lyot filter for dual-wavelength mode-locking and tunable single-wavelength mode-locking operations. The Lyot filter is operated via simple adjustment of the PC to allow both the polarization states and total birefringence to be changed at the same time, which results in tunability of the filter spectral transmission peak wavelengths and amplitude. We firstly propose using a section of tunable birefringent element to tune the transmission of Lyot filter. Even the change of the tunable element is 1000 times smaller than the birefringence of the PMF, it is enough to tune the transmission peaks of the Lyot filter. We believe this new tunable mechanism will open up new direction of much simpler dynamic Lyot filter. Here, through adjusting the PC, dual-wavelength mode-locking at 1535 nm and 1564 nm is achieved. Moreover, single-wavelength mode-locking laser with the tunability from 1532 nm to 1562 nm can be also obtained. Unlike other previously demonstrated Lyot-filter based lasers, this novel fiber-based Lyot-filter offer advantages for flexible dynamic tuning in one single laser cavity without the need to rebuild different laser cavities. We believe our proposed laser design with this novel filter can greatly simplify the set-up for dual-wavelength and wavelength tunable mode-locking of fiber laser, and is highly promising for various applications such as dual-comb spectroscopy, terahertz generation and fiber sensing.
Funding
Japan Society for the Promotion of Science ((S)18H05238, (B)19H02149).
Disclosures
No conflict of interest exits in the submission of this manuscript
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