Longitudinal Mode Hopping Research Articles

Theoretical analysis of the longitudinal mode behavior due to mode competition in bulk semiconductor lasers

The following typical longitudinal mode phenomena occur in bulk semiconductor lasers: single-mode or multimode operation for longitudinal modes, longitudinal mode suppression except for a dominant mode, asymmetry of the spectral profile of longitudinal modes, mode hopping, and dominant mode hysteresis. This article presents the theoretical occurrences of these phenomena in a unified manner by solving mode-competition equations, including nonlinear gain based on the density matrix method. We select various types of bulk semiconductor lasers for theoretical analyses, including an 850 nm GaAs material, 1.3 and 1.55 µm InGaAsP materials, and wide-striped and narrow-striped structures of an active region. These unified analyses reveal the main factors of these phenomena and the physical parameters that influence these phenomena. A good understanding of these physical parameters will help in designing bulk semiconductor lasers with the desired characteristics.

Single-Longitudinal Mode Ytterbium-Doped Fiber Laser with Ultra-Narrow Linewidth and High OSNR Using a Double-Ring Passive Subcavity

A continuous wave (CW) ultra-narrow linewidth single-longitudinal mode (SLM) ytterbium-doped fiber laser (YDFL) based on narrowband fiber Bragg grating (NB-FBG) and double-ring passive subcavity (DR-PS) was studied. The filtering characteristics of the double-ring passive subcavity are analyzed theoretically, and it is used as a high-precision mode filter to eliminate the dense longitudinal mode and mode hopping of YDFL and ensure that the laser operates in the SLM state. Experimental results show that the laser has a central wavelength of 1030.052 nm at room temperature, an optical signal-to-noise ratio of up to 73 dB, and an ultra-narrow linewidth of 355 Hz. In addition, we measured the short-term and long-term stability of the laser, with wavelength and power fluctuations of less than 0.008 nm and 0.19 dB, respectively, over 120 min. As a result, we obtain SLM YDFL with high stability, ultra-narrow linewidth, and a high optical signal-to-noise ratio.

Sub-kHz-linewidth continuous-wave single-frequency ring-cavity fiber laser based on high-gain Er: YAG crystal-derived silica fiber.

Throughout the development of single frequency fiber lasers (SFFLs), gain fiber is one of the most important components, which can greatly affect the quality of SFFLs. Here, we fabricated an Er: YAG crystal-derived silica fiber (EYDSF) using a CO2 laser-heating drawing technique, with a high gain coefficient of 1.74 dB/cm. Employing the EYDSF of only 10 cm as a gain medium, we constructed a continuous-wave ring-cavity SFFL with an all-fiber system. An ultra-narrow linewidth <660 Hz was achieved harnessing a homemade low-concentration Er-doped silica fiber as a saturable absorber. Importantly, the SFFL output power was up to 32.7 mW at 1560 nm. What's more, no multi longitudinal mode or mode hopping were found in 2 hours, and the fluctuation of power was <0.63% in 8 hours. Furthermore, the relative intensity noise was lower to -145 dB/Hz at frequencies over 1 MHz. The results indicate that the ring-cavity SFFL has desirable performance in output power, linewidth, stability and noise, which serves a prospective candidate applied to coherent optical communications, high-precision sensors, laser radars and other advanced fields.

A narrow-linewidth three-mirror VCSEL for atomic devices

We consider the use of a recently invented three-mirror vertical-cavity surface-emitting laser (VCSEL) as an optical-pumping light source for next-generation atomic devices (e.g., atomic clocks). While VCSELs are attractive given their very low size, weight, and power and intrinsic resistance to longitudinal mode hops, their relatively large linewidths give rise to efficient laser phase-noise to transmitted intensity noise conversion (PM-to-AM), which severely limits the signal-to-noise ratio in vapor-cell signals. In contrast, the three-mirror VCSEL has a linewidth more than ten times narrower than traditional VCSELs. Using a three-mirror VCSEL in a continuous-wave Cs vapor-cell atomic clock testbed, we demonstrate the utility of this laser for next-generation atomic devices in general, obtaining a short-term stability of 3.6 × 10−12/τ1/2 without any attention to PM-to-AM mitigation strategies.

A Method for Improving Light Intensity Stability of a Total Reflection Prism Laser Gyro Based on Series Correction and Feedforward Compensation

The instability of scale factor and null shift are closely related to the change of the light intensity of a total reflection prism laser gyro (TRPLG). To improve the light intensity stability, the light intensity stabilization principle of TRPLG was analyzed, and a mathematical model of light intensity stabilization system was established. Aiming at the problem that original system has a long adjustment time, a series correction method of adding a proportional integral differential (PID) link in the forward channel was adopted. According to the third-order optimal design, the original I-type third-order overdamped system was optimized to the new II-type third-order underdamped system, which ultimately improved the system's dynamic and steady-state performance. Feedforward compensation was used to introduce the light intensity disturbance during the longitudinal mode hopping into the closed-loop system, which realized the full compensation of the light intensity error and improved the light intensity characteristics during the longitudinal mode hopping. Combined with the above two methods, experimental results showed that the new system with feedforward compensation improved the light intensity stability by 32% at constant temperature and by 40% at variable temperature, with the TRPLG's bias stability improved by more than 16% at constant temperature and by 22% at variable temperature.

Theoretical investigation of a tunable external cavity diode laser based on a single cavity all-dielectric thin-film Fabry–Perot filter

The single cavity all-dielectric thin film Fabry–Perot filter (s-AFPF) has been theoretically investigated in this paper as a means of tuning the wavelength in an external cavity diode laser (ECDL), and the means of limiting longitudinal mode hopping has been also theoretically investigated. When a TE or TM plane wave irradiates an s-AFPF, a quasi-linear relationship is found in a certain wavelength range between the optical intensity peak transmittance wavelength of s-AFPF and the cosine value of plane wave incident angle at s-AFPF. Based on this feature, we proposed and theoretically investigated an ECDL configuration based on an s-AFPF. By theoretical calculation, the actuator flat edge against the steel ball may be replaced by a bent edge to convert the mode-hop wavelength region into mode-hop-free wavelength region. The ECDL can be used in the application of environmental monitoring, atomic and molecular laser spectroscopy research, precise measurements, and so on.

Intensity Noise Properties of Midinfrared Injection Locked Quantum Cascade Lasers: II. Experiments

We present experimental investigations of the intensity noise properties of injection-locked midinfrared emitting quantum cascade lasers. Following the theoretical approach by Simos <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , the injection locking is realized below and near the threshold of the free running slave laser, resulting in an efficient technique to achieve low-noise operation. We find that below the threshold, the locking characteristics as locking range, shape and bandwidth, are different in comparison with those above threshold. In addition, we also investigate injection locking into longitudinal side modes of the slave laser apart by several longitudinal mode hops, and observe similar characteristics, however, with the potential to achieve higher relative intensity noise suppression. The measurements are confirmed by additional numerical simulations with a new model, which considers the multimode spectrum of the slave laser and the spectral profile of the material gain. Under the actual experimental conditions, a reduction of the relative intensity noise of the slave laser of up to 10 dB (above threshold) and up to 20 dB (below threshold) in comparison to the free running slave laser noise level is achieved.

SCLC Degradation in 980 nm Pump Laser by Using Electrical Noise

The knowledge of the noise levels is important for pump laser diodes as it allows to study and to locate the noise sources and their origin. 980 nm fresh and aged pump lasers have been characterized by using electrical noise measurements. At 10 Hz, the spectra are dominated by () noise. Current noise spectral density (CNSD) is dominated by (). The trapping defect density near the n+n- and p+p- interfaces is related to pinching of the space-charge-limited current (SCLC) effect. An excess electrical noise due to longitudinal mode hopping is correlated with optical power fluctuations.

Theoretical analysis of the wavelength tuning characteristics in a tunable ECDL based on a s-AFPF

The single cavity all-dielectric thin film Fabry-Perot filter (s-AFPF) is investigated in this letter as a means of tuning the wavelength in an external cavity diode laser (ECDL), and the means of limiting longitudinal mode hopping is also investigated. When a TE or TM plane wave irradiates a s-AFPF, a quasilinear relationship is found in a certain wavelength range between the optical intensity peak-transmittance wavelength of s-AFPF and the cosine value of plane wave incident angle at s-AFPF. Based on this feature, we propose and investigate a configuration with a s-AFPF in a tunable ECDL. By theoretical calculation, a mode-hop-free wavelength tuning range of ∼5 nm around 1 550 nm is achieved. The ECDL can be used in the application of environmental monitoring, atomic and molecular laser spectroscopy research, precise measurements, and so on. OCIS codes: 140.2020, 140.3600, 310.6860. doi: 10.3788/COL201210.S11411. Tunable external cavity diode lasers (ECDLs) are playing an increasingly important role in coherent optical telecommunications, atomic and molecular laser spectroscopy, precise measurements, and environmental monitors [1] . Currently, two principal configurations in tunable ECDLs are the Littrow configuration [2−7] and the Littman–Metcalf configuration [8−11] and in both the

Investigation of a linearly tunable external cavity diode laser in a configuration with single cavity all-dielectric thin-film Fabry–Pérot filter

The single cavity all-dielectric thin film Fabry-Pérot filter (s-AFPF) has been investigated in this paper as a means of tuning the wavelength in an external cavity diode laser (ECDL), and the means of limiting longitudinal mode hopping has been also investigated. When a TE or TM plane wave irradiates an s-AFPF, a quasilinear relationship is found in a certain wavelength range between the optical intensity peak-transmittance wavelength of s-AFPF and the cosine value of plane wave incident angle at s-AFPF. Based on this feature, we proposed and investigated a configuration with an s-AFPF in an ECDL. By theoretical calculation, a mode-hop-free wavelength tuning range of about 8 nm around 1551.5 nm was achieved. Also, a method in further extending the width of the mode-hop-free wavelength region is proposed and discussed. The ECDL can be used in the application of environmental monitoring, atomic and molecular laser spectroscopy research, precise measurements, and so on.

Single-frequency fiber ring laser with 1W output power at 1.5 µm.

We report a single-frequency fiber laser with 1W output power at 1.5 mum which is to our knowledge, five times the highest power from a single-frequency fiber laser reported to-date. The short unidirectional ring cavity approach is used to eliminate the spatial gain hole-burning associated with the standing-wave laser designs. A heavily-doped phosphate fiber inside the ring resonator serves as the active medium of the laser. Up to 700mW of output power, the longitudinal mode hops have been completely eliminated by using the adjustable coupled-cavity approach. At higher power levels, the laser still oscillates at a single longitudinal mode, but with infrequent mode hops that occur at a rate of few hops per minute. Compared to the Watt-level single-frequency amplified sources, our approach is simpler and offers better noise performance.

Low-frequency intensity fluctuations in high-power single-mode ridge quantum-well InGaAs/AlGaAs heterostructure semiconductor lasers

It is shown that for an average output power of 60 mW, the spectral density of low-frequency intensity fluctuations in single-mode semiconductor lasers lies in the interval 6 × 10-17 — 10-15 W2 Hz-1. This fluctuation level is caused by the presence of subthreshold longitudinal modes and mode hopping, leading to the emergence of a noticeable 1/f component in their spectrum.

Longitudinal mode control of an erbium ring fibre laser containing an intracavity chirped Bragg grating etalon

We present preliminary data which show that it is possible to prevent longitudinal mode hops in a unidirectional erbium ring fibre laser by including an all fibre etalon formed from chirped fibre Bragg reflectors. A reduction in the relative intensity noise of ∼ 10 dB is observed when the laser is frequency stabilised. A resolution limited linewidth of 170 kHz is observed from the laser.

Polarization spectroscopy applied to the frequency stabilization of rare-earth-doped fiber lasers: a numerical and experimental demonstration

The frequency response of a unidirectional ring laser containing a linear polarizer and a weakly birefringent etalon is analyzed numerically. The data obtained indicate that by sensing the changes in the polarization state of the light that is coupled from the etalon, a suitable error signal can be derived that can be used to prevent longitudinal mode hops. This idea is verified experimentally by configuration of an erbium ring fiber laser with a nonplanar 3×3 fused-fiber coupler to form the intracavity etalon. When the longitudinal mode hops are inhibited and the oscillating frequency of the fiber laser is locked to a reference confocal etalon, the resulting radiation has a linewidth of 14 kHz with an associated rms frequency jitter of 2 kHz.

Anomalous longitudinal mode hops in GaAs/AlGaAs distributed Bragg reflector lasers

We investigate normal and anomalous longitudinal mode hops in GaAs/AlGaAs-based distributed Bragg reflector (DBR) lasers; anomalous mode hops are defined as those which move toward shorter wavelengths with increasing temperature, which is unexpected. The two-section DBR lasers discussed in this letter, consisting of a gain section and an unpumped Bragg reflector, typically exhibit one mode hop in a 10 K temperature range. Although the longer wavelength modes are expected to start lasing when raising device temperature, occasional mode hops to a shorter wavelength are seen. We derive a model for temperature-dependent wavelength tuning, with which the overheating of the gain section is described empirically. This model allows an accurate numerical simulation of both kinds of temperature-induced longitudinal mode hops.

Frequency stabilization of rare-earth-doped fiber lasers by use of 3 × 3 nonplanar fused-fiber couplers

We present a numerical analysis that indicates that a 3 x 3 nonplanar fused-fiber coupler can be used to inhibit longitudinal mode hops in a narrow linewidth unidirectional ring fiber laser. We also give preliminary experimental data that support our numerical analysis.

Theoretical model and experimental demonstration of frequency control in rare-earth-doped fiber lasers with a 3 × 3 nonplanar fused-fiber coupler

We present a numerical analysis that illustrates the potential of 3×3 nonplanar fused-fiber couplers for inhibiting longitudinal mode hops in rare-earth-doped-fiber lasers. A unidirectional erbium-doped ring-fiber laser is configured that contains a single 3×3 fused-fiber coupler. Narrow linewidth operation without any evidence of longitudinal mode hops is observed. The oscillating linewidth is 14 kHz with a rms frequency jitter ∼2 kHz. The relative intensity noise is measured in both the frequency stabilized and the frequency unstabilized cases. An average 15-dB reduction in the relative intensity noise over the 0–200 Hz frequency range is observed when the laser is stabilized.

Narrow linewidth operation of an erbium fiber laser containing a chirped Bragg grating etalon

A numerical model is presented which shows that a control signal suitable for preventing longitudinal mode hops can be derived by sensing changes in the polarization state of the light reflected from an intracavity etalon. This is verified experimentally by including a continuous fiber etalon formed from chirped Bragg reflectors in a unidirectional erbium ring fiber laser. Mode hop free operation is observed with resolution limited linewidths of 170 kHz.

Longitudinal mode control of a narrow-linewidth fiber laser by use of the intrinsic birefringence of the fiber laser

We present preliminary experimental data that show that the intrinsic birefringence in a ring erbium fiber laser containing a length of standard communications-grade optical fiber can be used to inhibit longitudinal mode hops. Mode-hop-free operation for periods in excess of several hours is observed with resolution-limited linewidths of 240 kHz.

Continuously tunable single-frequency erbium ring fiber laser

We report on a frequency-stabilized narrow-linewidth continuous-fiber erbium-doped ring fiber laser in which the operating wavelength is defined by an overlay filter. The oscillating frequency is stabilized by a side-of-fringe locking technique, and longitudinal mode hops are inhibited by phase-sensitive detection of the signal rejected from an intracavity all-fiber étalon. With this frequency-stabilization scheme resolution-limited linewidths of 68 kHz with a root mean square frequency jitter of 10 kHz are measured. The oscillating frequency is continuously tunable over 3 GHz.