Beat Spectrum Research Articles

Generation of longitudinal modes in an active Fabry-Perot fiber interferometer pumped by a low-power single-frequency semiconductor laser

The results of experiments on taking beat spectra at the exit of a stationary (modulator-free) fiber Mach-Zender interferometer are presented. The interferometer is excited by a single-frequency semicon-ductor laser with an output power varying from 0.65 to 2.00 mV and a line-width of less than 20 kHz. The observed line beat spectrum is explained based on the fact that the extended (several kilometers long) arm of the interferometer serves as an active fiber Fabry-Perot interferometer. In this case, the effects of enhancement of stimulated Mandel’shtam-Brillouin scattering in the fiber and the Fresnel reflection of light from the end faces of the fiber play a decisive role. Remarkably, these effects are observed at a low (less than 0.3 mW) power deposited into the fiber. The features of beat signals are considered, and the feasibility of applying this effect in high-sensitive field and displacement optical sensors is indicated.

Characterization of All-Optical Clock Recovery for 40 Gb/s RZ-OOK and RZ-DPSK Data Using Mode-Locked Semiconductor Lasers

We investigate the characteristics of all-optical clock recovery for return-to-zero differential phase-shift keying (RZ-DPSK) and return-to-zero on-off keying (RZ-OOK) signals at 40 Gb/s using mode-locked semiconductor lasers. Two mode-locked lasers with different configurations, a bulk active layer distributed Bragg reflector laser and a quantum-dot active layer Fabry-Perot laser, are used in the experimental investigation. The results reveal the implications of the input signal power, beat spectrum linewidth of the mode-locked laser, and intensity modulation on the jitter characteristics of the recovered clock signal.

Low-timing-jitter all-optical clock recovery for 40 Gbits/s RZ-DPSK and NRZ-DPSK signals using a passively mode-locked quantum-dot Fabry–Perot semiconductor laser

We investigate experimentally all-optical clock recovery for return-to-zero (RZ) and nonreturn-to-zero (NRZ) differential phase-shift keying (DPSK) signals at 40 Gbits/s using a passively mode-locked quantum-dot Fabry-Perot (QD-FP) semiconductor laser. The QD-FP laser exhibits a beat spectrum linewidth of 80 kHz, which enables a recovered clock signal with a root-mean-square timing jitter of 160 fs for the RZ-DPSK signal and 240 fs for the NRZ-DPSK signal. The timing jitter of the recovered clock signal is characterized for different values of the input signal power and the input signal optical signal-to-noise ratio.

Linewidth measurements of MEMS-based tunable lasers for phase-locking applications

We have used two techniques - the beat spectrum method and the frequency noise spectrum method - to measure linewidths of microelectromechanical systems (MEMS)-based tunable external cavity lasers (ECLs) in the C-band (1527-1567 nm). The two techniques yield similar results, but the latter is able to measure narrow linewidths more accurately in the presence of frequency jitter. The MEMS-based ECL linewidths are found to be inversely proportional to output powers over a wide range of powers. At output powers of 12 dBm, the beat linewidth is no more than 30 kHz, corresponding to per-laser linewidths of about 15 kHz. We show that these lasers are suitable for coherent detection of quadrature phase-shift keying.

Triple-Mode Action of a Broad-Band Laser from the Viewpoint of Intracavity Frequency-Dispersion Spectroscopy

For the beat spectrum of a triple-mode dye laser, there exists a component with a frequency equal to the difference in intervals between adjacent modes. The correlated technical fluctuations of these intervals compensate each other. This experimental fact is considered as promising for the development of a new spectroscopy method with a detection limit of the order of 10-10–10-11 cm-1 with respect to absorption in the probing volume. It is based on different changes in adjacent-mode intervals due to dispersion of the resonance line of the intracavity absorber. Thus the low frequency signal appearing in the beat spectrum can be detected. The latter is proportional to the density of absorbing particles.

270–360 GHz tunable beat signal light generator for photonic local oscillator

A new millimetre- and submillimetre-wave generation method is proposed. The method employs three techniques, namely the modulation of CW light by an electroabsorption modulator, pulse spectrum expansion through higher-order soliton compression, and spectrum filtering by using tunable fibre Bragg gratings. With this method, a continuously tunable beat signal from 270 to 360 GHz with no frequency fluctuation was obtained experimentally. In addition, the method requires only one laser source, and generates a beat spectrum with a linewidth of less than 10 Hz. By using optical filters with a sufficiently wide tuning range, it is possible to generate a beat signal at any frequency from 100 GHz to 1 THz.

Development of Phase-lock System between Two Single-Mode Lasers for Optical-Optical Double Resonance Spectroscopy

The difference frequency between two single-mode CW lasers was stabilized to a microwave (MW) frequency reference by using an optical phase-lock loop (PLL). The long-term frequency jittering of the beat spectrum was much suppressed by this method. The difference frequency was able to be continuously scanned from 0.1 to 18 GHz by changing the microwave reference frequency. It was also possible to scan the two lasers synchronously keeping the difference frequency constant. This stable and flexible light source would be an ideal light source for optical-optical double resonance (OODR) spectroscopy and resonance laser Raman experiments. For the demonstration, this new system was applied to an OODR experiment using a Rubidium atom. The hyperfine structures in the D2 line were observed in the sub-Doppler condition.

Analysis of the ST-T complex of the electrocardiogram using the Karhunen--Loève transform: adaptive monitoring and alternans detection.

The Karhunen-Loève transform (KLT) is applied to study the ventricular repolarisation period as reflected in the ST-T complex of the surface ECG. The KLT coefficients provide a sensitive means of quantitating ST-T shapes. A training set of ST-T complexes is used to derive a set of KLT basis vectors that permits representation of 90% of the signal energy using four KLT coefficients. As a truncated KLT expansion tends to favor representation of the signal over any additive noise, a time series of KLT coefficients obtained from successive ST-T complexes is better suited for representation of both medium-term variations (such as ischemic changes) and short-term variations (such as ST-T alternans) than discrete parameters such as the ST level or other local indices. For analysis of ischemic changes, an adaptive filter is described that can be used to estimate the KLT coefficient, yielding an increase in the signal-to-noise ratio of 10 dB (u = 0.1), with a convergence time of about three beats. A beat spectrum of the unfiltered KLT coefficient series is used for detection of ST-T alterans. These methods are illustrated with examples from the European ST-T Database. About 20% of records revealed quasi-periodic salvos of ischemic ST-T change episodes and another 20% exhibit repetitive, but not clearly periodic patterns of ST-T change episodes. About 5% of ischemic episodes were associated with ST-T alterans.

Beat spectrum of a dye laser generating three nonequidistant modes

The optical beat spectrum of a three-mode jet dye laser was investigated. When the modes became nonequidistant the combination tones shifted from their calculated frequencies, the intensity became modulated at the frequency of the deviation from the equidistant distribution, and the generated modes acquired side frequencies. The beat spectrum showed not only splitting of the fundamental frequencies, but was also enriched with components of frequencies which were single and double multiples of the difference between the intermode frequency intervals. Technical fluctuations of these differences, associated with an instability of the optical length of the empty cavity, were correlated and were compensated in the low-frequency difference components.

Coherent optical frequency domain reflectometry using phase-decorrelated reflected and reference lightwaves

We report a new scheme for coherent optical frequency domain reflectometry (C-OFDR) where the coherence length of the lightwaves does not limit the measureable fiber length. In this scheme, we use the beat spectrum which results when we mix reflected and reference lightwaves whose phases are not correlated. We demonstrated this scheme using a narrow-linewidth-lightwave source and an external electro-optical phase modulator. We measured Rayleigh backscattering and Fresnel reflections from a 30-Ion optical fiber, and achieved a spatial resolution of 5 m for two neighboring Fresnel reflectors located at the far end of the fiber. We estimated the expected spatial resolution and single-way dynamic range for our new scheme and show that it is capable of measuring long optical fibers with high-spatial resolution.

Observation of mode beating and self-frequency locking in a nearly single-mode semiconductor laser

The beat spectra between the adjacent modes of a semiconductor laser having no external cavity are reported. Passive beat-frequency locking is observed at high power without a saturable absorber. This self-induced locking is manifested by the dramatic narrowing (by four orders of magnitude) of the beat spectrum between the dominant and adjacent modes. It is accompanied by the transition from nearly single-mode behavior to a highly structured multifrequency optical spectrum. >

Effect of amplifier saturation on one-sided beat spectrum of driven unlocked oscillator

Amplifier saturation converts the nominal one-sided beat spectrum of a driven unlocked oscillator into a two-sided one of asymmetric amplitude. It is shown that this can be attributed to the conversion of the circularly polarised phasemodulation vector to one with an elliptical component.

Optical Amplification and Spontaneous Emission in an Ar + Discharge

The relationship between optical amplification and spontaneous emission noise has been studied in an argon ion travelling wave amplifier by beat spectrum analysis. Different optical arrangements have been examined with both single and double pass amplification. The results establish that when coherent light travels through a gain tube, the amplified beam acquires intensity fluctuation noise which is greater than that of a Poissonian distribution (shot noise). The noise power is shown to be the same as that of an equivalent unamplified beam mixed with the amplified spontaneous emission from the gain tube. It is shown that such measurements provide a practical technique for the otherwise difficult determination of the degree of population inversion in the gain medium. They also give insight into the potential of optical amplifiers for enhancement of lidar and system performance.

Novel method using white spectral probe signals to measure Brillouin gain spectra of pure silica core fibres

The beat spectrum of white spectral probe signals and a pump laser are used to directly give the Brillouin gain spectrum on an RF spectrum analyser. The measured peak gain coefficient is about 2.3 × 10−11m/W and the Brillouin linewidth is 29 MHz which can be further broadened by the additional strain distribution.

Frequency stabilization of laser diodes using 1.51-1.55 mu m absorption lines of /sup 12/C/sub 2/H/sub 2/ and /sup 13/C/sub 2/

Frequency stabilization of a 1.55 mu m DFB laser diode is demonstrated using vibrational-rotational absorption of /sup 12/C/sub 2/H/sub 2/ and /sup 13/C/sub 2/H/sub 2/ molecules (named VRAM). Frequency stability within 2 MHz peak to peak fluctuation can be achieved in the wavelength region of 1.51-1.55 mu m. Frequency-stabilized DFB laser compact modules have been constructed. Frequency stabilities are evaluated by measuring the beat spectrum of the two lasers. In addition, the temperature and pressure dependences of the acetylene absorption lines are characterized. >

Small-frequency-difference stabilization of laser diodes using /sup 12/C/sub 2/H/sub 2/ and /sup 13/C/sub 2/H/sub 2/ absorption lines for transmitter and local oscillator of optical heterodyne systems

Small-frequency-difference stabilization of distributed Bragg reflector (DBR) laser diodes has been demonstrated for the first time by using vibrational-rotational absorption of /sup 12/C/sub 2/H/sub 2/ and /sup 13/C/sub 2/H/sub 2/ molecules. Frequency stabilization of two DBR laser diodes has been carried out at a wavelength of about 1.536 mu m. The frequency difference between the two stabilized lasers was evaluated to be 9 GHz by the beat spectrum measurement. The experimental results suggest that such a pair of frequency-stabilized laser diodes with a small frequency difference could be used as the transmitter and local oscillator of optical heterodyne systems. >

0.04 Hz relative optical-frequency stability in a 1.5 mu m distributed-Bragg-reflector (DBR) laser

The optical frequency of a 1.5- mu m distributed-Bragg-reflector (DBR) laser is stabilized against that of a master laser by heterodyne-type frequency locking with a phase-locked loop (PLL). Despite the laser's wide linewidth of 16 MHz, stable PLL operation with an optical hold-in range of 26 GHz is realized, and residual frequency fluctuations are reduced to 0.04 Hz at an averaging time of 500 s. The combination of DBR laser and PLL is, therefore, suitable for frequency-controlled light sources. The offset error from the settled frequency caused by the bandlimited beat spectrum is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency stabilization of 1.55 mu m DFB laser diode using vibrational-rotational absorption of /sup 13/C/sub 2/H/sub 2/ molecules

The vibrational-rotational absorption of /sup 13/C/sub 2/H/sub 2/ molecules (VRAMs) in the 1.52-1.55 mu m region was investigated in detail. On the basis of this investigation, frequency stabilization of 1.55- mu m distributed-feedback (DFB) laser diodes was demonstrated. Frequency stability to within 2 MHz peak/peak fluctuation was achieved at the 1.54949- mu m wavelength. In addition, frequency stabilizations in the wavelength regions of 1.53 mu m and 1.54 mu m were also carried out by using the strong absorption lines of /sup 13/C/sub 2/H/sub 2/ VRAM in these wavelength regions. The spectral width of the frequency-stabilized DFB laser diode was found to be 25 MHz. The absolute frequency was found to be stabilized at least to within 25 MHz by the measurement of beat spectrum.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Tunable cw color-center laser emitting two neighboring longitudinal modes

A two-mode operating regime was achieved using a cw color-center laser with an intermode frequency spacing of 150 MHz. The intermode beat signal was recorded. The width of the beat spectrum did not exceed 2 kHz. The beat signal was used to lock the length of the laser resonator and the intracavity interferometer.

Spectral characteristics for 1.3 μm monolithic external cavity DFB lasers

Spectral characteristics for 1.3μm monolithic external cavity (MEC) DFB lasers have been studied experimentally. As drive current increased, light output and spectral linewidth were changed periodically. Less than 2 MHz linewidths were obtained at the minimum points of each period. The narrowest beat spectrum linewidth was less than 3MHz with two MEC DFB lasers.