Single Longitudinal Mode Output Research Articles

Tunable and switchable dual-wavelength SLM narrow-linewidth fiber laser with Lyot filter cascaded by double-ring cavity

An erbium-doped fiber (EDF) laser with single longitudinal mode (SLM), narrow linewidth, and switchable dual-wavelength based on a Lyot filter cascaded dual-ring cavity structure (DRCs) has been designed and constructed. In the experimental system, an Employed passive DRCs enhances the longitudinal mode spacing, while the Lyot filter effectively mitigates mode competition within the laser cavity. The composite filter structure facilitates stable dual-wavelength output while ensuring a consistent SLM output. Then, experimental results reveal optical signal-to-noise ratios (OSNR) with 62 dB for single-wavelength and 55 dB for dual-wavelength. Additionally, the power and wavelength fluctuations for both single-wavelength and dual-wavelength remained stable throughout monitoring periods of 20 min and 1 h, respectively. Meanwhile, a maximum tuning range of 2 nm is achieved by stretching a polarization-maintaining fiber Bragg grating (PM-FBG) using a fiber displacement stage. The linewidths of each wavelength are 2.8 kHz and 2.6 kHz, respectively. During the process of wavelength tuning, the fluctuations of linewidth do not exceed 0.3 kHz and 0.45 kHz, respectively. Simultaneously, the polarization states of each wavelength are measured, which reveals orthogonal single polarization. The relative intensity noise (RIN) is measured below −133 dB/Hz at frequencies above 1 MHz.

Narrow linewidth fiber laser using compound-ring cavity filter and self-injection locked mechanism

Narrow linewidth erbium-doped fiber lasers (EDFLs) hold in significant potential optical fiber sensing, wavelength multiplexing transmission, and optical fiber communication. However, many EDFLs produced outputs with linewidth in several kHz level. Therefore, a high optical signal-to-noise ratio (OSNR), narrow linewidth single-longitudinal-mode (SLM) EDFL was proposed and investigated in this work. The EDFL utilized a combination of compound-ring cavity filter and a self-injection locked mechanism to generate a stable SLM output with an OSNR greater than 72 dB. The maximum wavelength drift and maximum peak power fluctuation over 60 min were maintained at low values of 0.0041 nm and 0.16 dB. Moreover, a narrow linewidth of 1.82 kHz at a measuring time of 0.002 s was realized. Furthermore, the research also investigated output power variations with different pump powers, relaxation oscillation, and the dependence of output central wavelength on the axial strain.

Highly stable single-longitudinal-mode erbium-doped fiber laser using dual-ring compound cavity filter and saturable absorber

Single-longitudinal-mode (SLM) fiber lasers have the advantages of stable output, long coherence length, and low noise. However, several fiber lasers still maintain multiple longitudinal mode outputs. Thus, a highly stable SLM erbium-doped fiber laser (EDFL) was proposed and investigated. The dual-ring compound cavity (DRCC) filter, composed of two Type-Ⅱ fiber rings, and a 1.5 m unpumped erbium-doped fiber (EDF) was used as a mode-selection filter. The mode-selection capability of the DRCC filter and unpumped EDF was investigated and discussed theoretically and experimentally. The proposed EDFL could generate stable SLM output with an optical signal-to-noise ratio of ∼ 64 dB over 60 min. The maximum wavelength drift was not observed on an optical spectrum analyzer and the maximum peak power fluctuation was kept at a low level of 0.16 dB. The output power variations with different pump powers, the relative intensity noise, the relaxation oscillation, and the linewidth at different integration times were investigated. Furthermore, the performance of the proposed EDFL acted as a strain sensor and temperature sensor with sensitivities of 1.0 pm/με and 11.4 pm/℃ were also discussed.

Thulium-doped fiber laser with bidirectional single-longitudinal-mode output using a cascaded triple-ring cavity filter

Single-longitudinal-mode (SLM) fiber lasers play important roles in optical fiber communication. However, there has been little reported research on bidirectional SLM output, especially in the 2-μm band. In the work reported here, a bidirectional SLM output thulium-doped fiber laser (TDFL) using a cascaded triple-ring cavity (CTRC) filter was proposed and investigated. To realize bidirectional output, the proposed TDFL had a simple structure, using only two 2 × 2 optical couplers. To realize SLM output, the transmission spectrum, as well as the reversibility of the CTRC filter were discussed and investigated in detail. The direction-switchable SLM output was realized by polarization-dependent loss. By tuning the polarization controller carefully, each direction could generate SLM output with an optical signal-to-noise ratio (OSNR) > 72 dB. The maximum wavelength and peak power fluctuation over 60 min were, respectively, 0.01 nm and 0.48 dB. The variation of output power with pump power, linewidth, relaxation oscillation, and relative intensity noise were also investigated. To the best of our knowledge, this is the first time that a sub-ring cavity filter has been used to realize bidirectional SLM output.

Dual-ring parity-time symmetric brillouin fiber laser with an unbalanced polarization Mach-Zehnder interferometer.

A dual-ring parity-time (PT) symmetric Brillouin fiber laser (BFL) with an unbalanced polarization Mach-Zehnder interferometer (UP-MZI) is proposed and experimentally investigated. An UP-MZI consisting of optical coupler, polarization beam combiner (PBC) and two asymmetric length arms with 10 km and 100 m single-mode fiber, is used to achieve Vernier effect and PT symmetry. Due to the orthogonally polarized lights created in the PBC, the dual-ring PT symmetry BFL with an UP-MZI implements two unbalanced length feedback rings that are connected to one another, one long length ring with a Brillouin gain and the other short length ring with a loss of the same magnitude, to break a PT symmetric and maintain the Vernier effect. By contrast with existing PT symmetry BFL studies, this design does not require same lengths of the gain and loss loops, but can manipulate freely PT symmetry status in accordance with a rational scaling factor between them. Experimental results reveal that the 3-dB linewidth of dual-ring PT symmetry BFL with an UP-MZI is about 4.85 Hz with the threshold input power of 9.5 mW, in accordance with the 97 Hz measured linewidth at the -20 dB power point. Within 60 mins of the stability experiment, the power and frequency stability fluctuation are ±0.02 dB and ±0.137 kHz, respectively. Thanks to the two asymmetric ring lengths, the sidemode suppression ratio (SMSR) is optimized by 54 dB compared to that with the only long ring structure, 26 dB when using only the Vernier effect or 12 dB for existing PT symmetry BFL. This BFL design with single longitudinal mode and high SMSR output can be applied to high coherent communication and Brillouin-based microwave photonics systems with low phase noise.

Monolithically Integrated Tunable Three- Section All-Active DBR Laser Diodes With 30 nm Tuning Range

We present a monolithically integrated three-section all-active AlGaInAs/InP tunable distributed Bragg reflector (DBR) ridge waveguide laser diode that uses thermal tuning mechanism and mode-hopping tuning principle to achieve wide-range wavelength tuning. It is similar to the traditional three-section DBR laser structure, including a gain section, a phase section, and a DBR grating section. The difference is that the phase section and the DBR section incorporate the same active quantum well structure as the gain section. In this case, weak effective refractive index changes of the three active sections affected by the current injection will occur since the carrier density is clamped by the threshold gain condition of the DBR laser diode. Despite this limitation, a noncontinuous tuning up to ∼30 nm (1564–1594 nm) can be observed using only current injection at a constant temperature of 25 °C. Significant thermal effects occurred during the tuning process. In addition, a discontinuity is introduced in the tuning because it does not contain a passive region for phase adjustment. However, the device still provides a stable single longitudinal mode output with nine channels, a wavelength spacing of about 4 nm, and a side mode suppression ratio (SMSR) of more than 30 dB. Furthermore, the DBR laser diode can be easily fabricated utilizing the conventional distributed feedback laser diode fabrication process without secondary epitaxy and heater metal. The all-active DBR laser diode will be expected to be applied in the coarse wavelength division multiplexing (CWDM) systems with the advantage of low cost.

Secondary Raman and Brillouin mode suppression in two- and three-mirror-cavity diamond Raman lasers.

We report an investigation into secondary mode suppression in single longitudinal mode (SLM) 1240 nm diamond Raman lasers. For a three-mirror V-shape standing-wave cavity incorporating an intra-cavity LBO crystal to suppress secondary modes, we achieved stable SLM output with a maximum output power of 11.7 W and a slope efficiency 34.9%. We quantify the level of χ(2) coupling necessary to suppress secondary modes including those generated by stimulated Brillouin scattering (SBS). It is found that SBS-generated modes often coincide with higher-order spatial modes in the beam profile and can be suppressed using an intracavity aperture. Using numerical calculations, it is shown that the probability for such higher-order spatial modes is higher for an apertureless V-cavity than in two-mirror cavities due its contrasting longitudinal mode-structure.

Bidirectional Single Longitudinal Mode Er-Doped Fiber Ring Laser Using a Bandpass Filter and Cascaded Fiber Ring Filter

Bidirectional single longitudinal mode fiber ring laser have advantages and potential in optical sensing, especially angular velocity sensing. The current methods for implementing bidirectional single longitudinal mode fiber ring laser cannot ensure complete reciprocity in the optical path, and the side mode suppression ratio needs to be increased, while the laser linewidth is relatively wide. This paper proposes a combined filtering structure using a bandpass filter and a cascaded add-drop type fiber ring filter. When realizing bidirectional single longitudinal mode output, the optical path structure is fully reciprocal including polarization state reciprocity, while the side mode suppression ratio is relatively high. The linewidth of the bidirectional laser is around 1.1 kHz, far lower than the previous report of tens of kHz. The fully reciprocal combined filter structure proposed in this paper does not require special devices, and its theoretical system is mature, which is conducive to the subsequent application and improvement of bidirectional single longitudinal mode fiber ring laser.

Narrow linewidth parity-time symmetric Brillouin fiber laser based on a dual-polarization cavity with a single micro-ring resonator.

A narrow linewidth parity-time (PT) symmetric Brillouin fiber laser (BFL) based on dual-polarization cavity (DPC) with single micro-ring resonator (MRR) is proposed and experimentally investigated. A 10 km single-mode fiber provides SBS gain, while a DPC consisting of optical coupler, polarization beam combiner and a MRR, is used to achieve PT symmetry. Due to the reciprocity of light propagation in the MRR, the PT symmetry BFL based on DPC implements two identical feedback loops that are connected to one another, one with a Brillouin gain coefficient and the other with a loss coefficient of the same magnitude, to break a PT symmetric. Compared with existing BFL studies, this design does not call for frequency matching of compound cavities structures or without ultra-narrow bandwidth bandpass filters. In the experiment, the 3-dB linewidth of PT symmetry BFL based on DPC with single MRR is 11.95 Hz with the threshold input power of 2.5 mW, according to the measured linewidth of 239 Hz at the -20 dB power point. And a 40 dB maximum mode suppression ratio are measured. Furthermore, the PT symmetry BFL's wavelength is tuned between 1549.60 and 1550.73 nm. This design with single longitudinal mode output can be applied to high coherent communication systems.

Nanosecond pulsed single longitudinal mode diamond Raman laser in the 1.6 µm spectral region.

We demonstrate the first nanosecond pulsed single longitudinal mode (SLM) intracavity-pumped diamond Raman laser, to the best of our knowledge. The eye-safe coherent source at 1634 nm, which was converted from the actively Q-switched 1342 nm Nd:YVO4 laser, yielded 4.35 W of multimode average output power with a pulse duration of 6 ns and peak power of 29 kW. By exploiting the spatial hole burning free gain mechanism in the Raman media, stable SLM operation was observed at low output power (0.46 W) for the free-running case. Furthermore, by incorporating an etalon in the fundamental standing-wave cavity, the spectral linewidth of the fundamental field was suppressed substantially below the diamond Raman gain linewidth and slightly less than the free spectral range of the mm-scale Raman resonator. Thereby, a much higher SLM output power of 1.63 W was obtained with a pulse duration of ∼9 ns and a spectral linewidth of ∼77 MHz.

Switchable single- and dual-wavelength thulium/holmium co-doped fiber laser with single-longitudinal mode output

A single-longitudinal-mode ring thulium/holmium co-doped fiber laser (THDFL) possessing the switchable single- and dual-wavelength outputs is proposed with two subring cavities and two polarization-maintaining fiber Bragg gratings. The switching operation is realized by tuning the pump power and polarization controller together. When the THDFL working at single-wavelength output state, the optical signal-to-noise ratio (OSNR) is ∼51 dB, and the maximum power and wavelength fluctuation within 1 h are 0.804 dB and 0.04 nm, respectively. Using the phase noise analysis-based linewidth measurement system, the linewidth is measured to be 0.16 MHz with a measuring time of 0.002 s. When the THDFL working at dual-wavelength output state, the OSNR is ∼55 dB, and the maximum power and wavelength fluctuation within 1 h are 0.802 dB and 0.05 nm, respectively. The proposed THDFL may find wide applications in microwave photonics generation and optical fiber laser sensor.

Sub‐nanosecond, single longitudinal mode laser based on a VBG‐coupled EOQ Nd:YVO4 oscillator for remote sensing

AbstractHigh energy, sub‐nanosecond, single longitudinal mode (SLM) pulse is especially preferred for LiDAR application. In this paper, a kHz‐level sub‐nanosecond electro‐optical Q‐switched Nd:YVO4 laser of SLM operation for remote sensing was demonstrated. By utilizing a compact resonator with a Volume Bragg Grating as output coupler, maximum SLM output power of 1.08 W and pulse width of 919 ps was obtained at 10 kHz. Moreover, we proposed a theoretical model to calculate the multimode threshold of the VBG‐coupled cavity. The longitudinal mode characteristics at four different cavity lengths were investigated experimentally and the results match well with the numerical simulations, which validates the theory. We expected the method could help to guide VBG‐coupled cavity design and construction.

High efficiency tunable unidirectional single-longitudinal-mode Er:YAG ring laser based on an acousto-optic modulator

A wavelength tunable single-longitudinal-mode (SLM) Er:YAG ring laser around 1.6 µm is demonstrated. By using an acousto-optic modulator (AOM) to force unidirectional operation, up to 10.4 W and 8.7 W SLM laser output power are obtained at 1645.22 nm and 1617.33 nm, with corresponding slope efficiencies of 45% and 40%, respectively. Besides, stable dual-wavelength operation at both 1645 nm and 1617 nm is also achieved with the maximum power of 9.1 W. By rotating the birefringent filter (BRF) in the ring cavity, the wavelength could be tuned from 1616.77 nm to 1617.51 nm and 1644.51 nm to 1646.12 nm. The line width is measured to be 125 kHz at 1617 nm and 131 kHz at 1645 nm via the time-delayed self-heterodyne method. As far as we know, 8.7 W is the highest continuous-wave SLM output power at 1617 nm.

Tunable DFB laser diode based on high-order surface isolation grooves working at 905nm

A widely tunable distributed feedback Bragg (DFB) laser diode based on surface isolation grooves has been produced by i-line lithography. Periodic p-electrodes on the mesas realize a single longitudinal mode output. The maximum continuous wave output power of the uncoated device was 145.3 mW/facet at 500 mA, and the maximum side-mode suppression ratio was over 37 dB. Experimentally, the laser diode could be tuned from 899.9 to 907.7 nm from 15 ∘C to 35 ∘C. The 3 dB linewidth was 920 MHz at 20 ∘C for an injected current of 300 mA. The method of fabricating the tunable DFB laser diode has great potential for many applications, such as in light detection and ranging (LiDAR) devices.

An Erbium Fiber Laser With Single-Frequency Oscillation and Wavelength-Upconverted Output

In this paper, we present experimentally an erbium-doped fiber (EDF) laser system to accomplish the single-longitudinal-mode (SLM) and wavelength-upconverted output by using nonlinear effect. To accomplish the SLM output, a short length of unpumped EDF saturable absorber (SA) is applied inside the EDF laser cavity for filtering densely multi-mode noise. In accordance with the designed EDF laser system, the lasing wavelength can be upconverted to a longer wavelength position. In addition, the output characteristics of output power, optical signal to noise ratio (OSNR), tunability, stability and linewidth in the EDF laser are also discussed.

Utilizing self-injection Rayleigh backscattering feedback for channel-selected erbium laser with single-longitudinal-mode output

To achieve the target of single-longitudinal-mode (SLM) operation in erbium-doped fiber (EDF) laser, a self-injected Rayleigh backscattering (RBS) feedback schematic is proposed and demonstrated experimentally. Based on the proposed RBS effect, the multi-longitudinal-mode (MLM) noises of the RBS EDF laser can be suppressed fully. Here, the output powers of −7.5 to −5.1 dBm are detected over the tuning bandwidth of 1523.0 to 1563.0 nm. Moreover, the optical signal to noise ratios (OSNRs) of 30.9 and 33.1 dB is also attained. In this study, the SLM oscillation and output instability of the EDF laser are also analyzed and discussed.

Mode evolution mechanism of Pr3+:YLF single longitudinal mode laser

We develop a theoretical model of multimode rate equation that accurately describes mode evolution mechanism of prelase Q-switched Pr3+:YLF single longitudinal mode (SLM). We find that it is important to consider the effect of gain difference when calculating the mode competition time accurately. We identify that there is an optimal parameter for the two step signal of acoustic-optic modulator which determines the stability of SLM output. Then, a visible light direct oscillation Q-switched SLM Pr3+:YLF laser at 639.5 nm is obtained, and the experimental results is agree with the simulation very well.

A single-frequency 1064-nm Yb[formula omitted] -doped fiber laser tandem-pumped at 1018 nm

In this study, a single-frequency 1064-nm Yb fiber laser tandem-pumped at 1018 nm is demonstrated for the first time. The single-frequency operation of 1064 nm laser is achieved by means of a unidirectional ring cavity together with an unpumped active fiber served as a saturable absorber (SA). A maximum single longitudinal mode (SLM) output power of 231 mW was obtained under the 1018-nm pump power of 1.4 W. The signal-to-noise ratio, spectral linewidth, and relative intensity noise were 72 dB, 1.4 kHz, and –141 dB/Hz, respectively. The spectral linewidth of 1064-nm single-frequency output under 1018-nm pumping was narrowed significantly in comparison with that under 976-nm pumping. Additionally, the laser performance under single- and multi-longitudinal-mode (MLM) 1018-nm pump light was investigated. The result revealed that the SLM pump helped improve the behavior of noise.

Feedback-Injected Erbium Fiber Laser With Selectable Tunability and Constant Single-Longitudinal-Mode Characteristic

In this investigation, a wavelength-selected single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser utilizing C-band EDF based gain medium over a tuning bandwidth of 1524.0 to 1573.0 nm is demonstrated experimentally. A self-injection Rayleigh backscattering (RB) is designed in the EDF ring laser configuration to suppress the multi-longitudinal-mode (MLM) oscillation and narrow the obtained linewidth. Here, the physical output features of the proposed self-injected RB EDF laser are also studied and discussed, such as the optical signal to noise ratio (OSNR), output power, and laser linewidth, respectively. Therefore, the demonstrated EDF laser not only can reach SLM output, but also can produce a flat output spectrum of 1524.0 to 1554.0 nm with 0.2 dB power variation. Moreover, the measured fluctuation of each wavelength can be kept at ±0.05 nm based on the presented self-injected RB EDF laser, while the uncertainty issue is also considered.

Single longitudinal mode external cavity blue InGaN diode laser

We present a 445-nm InGaN blue diode laser in wavelength-tunable Littrow-type external cavity with a single longitudinal mode output, and a linewidth of 4.7 MHz. Its tuning range was estimated at 4 nm. The maximum output power was up to 20 mW, and the slope efficiency was approximately 0.36 W/A. The external cavity-diode laser linewidth was measured as 4.7 MHz with a lasing threshold to 8.1 MHz at higher applied current. The slope of the linewidth broadening with applied currents was calculated as 57 kHz/mA. And the wavelength tuning range was approximately 4.3 nm at low applied current and narrowed to approximately 2.6 nm at high applied current. A blue external cavity diode laser can deliver a single frequency beam within entire operation range.