Intermode Frequency Research Articles

Active mode-locking in semiconductor lasers

This paper presents the results of experimental studies of active mode-locking in an external-cavity semiconductor laser. Two methods of obtaining active mode-locking regimes have been studied: by modulation from an external generator with a frequency close to the intermode frequency of the external laser cavity and by laser current modulation at the frequency of intermode beats of the external laser cavity through an amplification feedback loop. In the first case, a stability of intermode beats of 10-14 is experimentally achieved, and in the second case, it is shown that semiconductor laser current modulation by an amplified intermode beat signal from the output of a photodetector provides a stable mode-locking regime also with mode frequency tuning, i.e., a mode-locking regime involving intermode frequency tuning with optical cavity tuning. The effects that accompany mode locking, in particular, the pulling effect and the stability of the mode-locking regime, and possibility of obtaining highly stable intermode beats by suppressing the amplitude noise component in the modulating signal have been studied. A phenomenological study of the pulling and phase locking of the intermode frequency is attempted.

Method of improving the frequency repeatability of the intensity stabilized HeNe laser

A method of improving the frequency repeatability of a two-mode helium–neon laser starting from different initial conditions has been developed. The approach is based on locking the laser when a certain number of laser modes is observed. In order to avoid ambiguity we propose to tie the initial number of laser modes to the value of the intermodal frequency. Experiments proved that with the suggested design it is possible to lock the laser on the desired mode regardless of the initial conditions. In the experiments the frequency repeatability of the laser was equal to its frequency stability.

Superthin resonator dye laser with THz intermode frequency separation

Two-color laser irradiation is considered an effective way to pump THz excitations for numerous scientific and applied goals. We present a design for convenient laser source with THz intermode frequency separation. The setup is based on dye laser with superthin resonator pumped by a subnanosecond pulse laser. It was proven that the superthin resonator dye laser is useful, possesses high stability and high energy conversion, and generates narrow laser modes. The ability of this laser to pump CARS processes for THz vibrations is demonstrated.

Wide-range tunability, thermal locking, and mode-crossing effects in Kerr optical frequency combs

We theoretically and experimentally investigate some effects related to the Kerr optical frequency comb generation, using a millimeter-size magnesium fluoride ultrahigh quality disk resonator. We show that the Kerr comb tunability can be extremely wide in the Turing pattern (or primary comb) regime, with an intermodal frequency that can be tuned from 4 to 229 multiple free spectral ranges (corresponding to a frequency spacing ranging from 24 GHz to 1.35 THz). We also discuss the role played by thermal locking while pumping the res- onator, as well as the effect of modal crossing when broadband combs are generated. © 2014 Society of Photo-Optical

Intermodal beat frequency stability in a polarization stabilized HeNe laser with an optical switch

We investigated the behaviour of a frequency stabilized 1mW two mode helium-neon laser incorporating an optical switch. In the paper we compare the stability of the intermodal frequency when the error signal is taken either from main or the parasite output of the tube. The possibility of using the constructed laser in the non-interferometric optical measurement device working with precision in the range of 1ppm is given. Full Text: PDF References K. Seta, T. O'Ishi, "Distance meter utilizing the intermode beat of a He–Ne laser", App. Opt. 29(3), 354 (1990). CrossRef T. Araki, S. Yokoyama, N. Suzuki, "Simple optical distance meter using an intermode-beat modulation of a He-Ne laser and an electrical-heterodyne technique", Rev. Sci. Instr. 65(6), 1883 (1994). CrossRef J. Rzepka, J. Pieńkowski, H. Pawołka, S. Sambor, "Two-frequency laser interferometer with phase shift measurement", Opt. App. XXVII(4), (1997). S. Yokoyama, T. Araki N. Suzuki, "Intermode beat stabilized laser with frequency pulling", App. Opt. 30(3), 358 (1994). CrossRef Wysocki G., Abramski K.M., "Frequency stabilisation of twin orthogonal mode He-Ne laser", Opt. App., vol. XXXII, no. 1-2, pp. 207-213, (2002) DirectLink Rzepka J., Maltese P., "Frequency stabilization of Zeeman He-Ne laser using a surface-stabilized ferroelectric liquid crystal", Proc SPIE, vol. 3186, pp. 162-166, 1997 CrossRef Wang. L., et al., "Laser Frequency Stabilization Based on Control of Intermode Beat Note", Chinese Journal of Lasers 34 (9) , pp. 1198-1202, 2007 DirectLink Kim M.-S., Kim S.-W., "Two-Longitudinal-Mode He-Ne Laser for Heterodyne Interferometers to Measure Displacement", App. Opt. 41(28), 5938, 2002 CrossRef Budzyń G., "Optokomutacyjna stabilizacja lasera dwumodowego", PhD Thesis, Wrocław University of Technology, Wrocław, 2006 Anishenko M. L., et al, "Natural Intensity and Intermode Beating Frequency Fluctuations in Two-Mode He-Ne and He-Ne/CH 4 Lasers", Las. Phys., Vol. 1, No. 5, pp. 511-534, 1991 DirectLink Lindberg A., "Mode frequency pulling in He–Ne lasers", Am. J. Phys. 67 (4), pp. 350-353 CrossRef T. Yoshino, "Reflection Anisotropy of 6328 A Laser Mirrors", Japn. J of App. Phys., vol. 18, no. 8, pp. 1503-1507 CrossRef Podżorny T., Budzyń, G., Rzepka, J., ICMEM 2012 (2012), pp. 137-140 J.Y. Yeom and T.H. Yoon, "Three-longitudinal-mode He-Ne laser frequency stabilized at 633 nm by thermal phase locking of the secondary beat frequency", App. Opt. 44, 266-270 (2005) CrossRef

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

N–H Stretching Modes of Adenosine Monomer in Solution Studied by Ultrafast Nonlinear Infrared Spectroscopy and Ab Initio Calculations

The N-H stretching vibrations of adenine, one of the building blocks of DNA, are studied by combining infrared absorption and nonlinear two-dimensional infrared spectroscopy with ab initio calculations. We determine diagonal and off-diagonal anharmonicities of N-H stretching vibrations in chemically modified adenosine monomer dissolved in chloroform. For the single-quantum excitation manifold, the normal mode picture with symmetric and asymmetric NH(2) stretching vibrations is fully appropriate. For the two-quantum excitation manifold, however, the interplay between intermode coupling and frequency shifts due to a large diagonal anharmonicity leads to a situation where strong mixing does not occur. We compare our findings with previously reported values obtained on overtone spectroscopy of coupled hydrogen stretching oscillators.

Absolute Angular Displacement Determination Based on Laser-Frequency Splitting Technology

We introduce an angle measurement system based on laser-frequency splitting technology, which is capable of absolute angular displacement determination. The core part is a dual-frequency microchip Nd:YAG laser which has two quarter-wave plates in the laser resonator and outputs two orthogonally linearly polarized lights with variable intermode frequency. A rotation of one of the wave plates shifts the frequency difference between modes. This angular displacement can thus be examined by detecting the shift of the frequency difference. The principle of the dual-frequency laser is demonstrated. A setup developed to accomplish angular displacement determinations and the experimental results of using this setup are then presented.

A Q-switched, mode-locked fiber laser employing subharmonic cavity modulation

We present a new and simple approach for the generation of Q-switched, mode-locked pulses from a laser cavity. The approach is based on cavity loss modulation that employs a subharmonic frequency of the fundamental intermode frequency spacing. A range of experiments have been carried out using an erbium-doped fiber-based ring cavity laser in order to verify that this simple approach can readily produce high quality Q-switched, mode-locked pulses. An active tuning of the Q-switched envelope repetition rate is also shown to be easily achievable by adjusting the order of the applied subharmonic frequency.

Multipoint sensor based on fiber Bragg gratings

In some control and industrial measurement systems of physical variables (pressure, temperature, flow, etc) it is necessary one system and one sensor to control each process. On the other hand, there are systems such as PLC (Programmable Logic Control), which can process several signals simultaneously. However it is still necessary to use one sensor for each variable. Therefore, in the present work the use of a multipoint sensor to solve such problem has been proposed. The sensor consists of an optical fiber laser with two Fabry-Perot cavities constructed using fiber Bragg gratings (FBG). In the same system is possible to measure changes in two variables by detecting the intermodal separation frequency of each cavity and evaluate their amplitudes. The intermodal separation frequency depends on each cavity length. The sensor signals are monitored through an oscilloscope or a PCI card and after that acquired by PC, where they are analyzed and displayed. Results of the evaluation of the intermodal frequency separation peak amplitude behavior with FBG stretching are presented.

Study of mode locking in a microwave-pumped diode laser close to the generation threshold

Active mode locking is studied in a diode laser with a three-mirror resonator upon the microwave modulation of the pump current. The mode-locking region with the minimal width of the spectrum of intermode beats is found, when the microwave frequency is close to the intermode frequency of an external resonator. This region is shown to be located close to the threshold pump current.

Stability and self-stabilisation of single-frequency lasing in a semiconductor laser

The problem of stability of single-frequency lasing is considered using a model including one lasing mode and two nearest subthreshold modes. It is shown that, the parametric interaction of the laser and subthreshold modes, due to the electron concentration beats at the intermode frequency, can cause the self-stabilisation of the single-frequency lasing regime, in which the laser frequency detuning with respect to the spectral gain maximum can exceed the mode interval, and spectral hysteresis can be observed.

Study of the spectral characteristics of a femtosecond Ti:sapphire laser after propagation of its radiation through a tapered fibre

The method and results of precision measurements of the intermode frequency of a Ti:sapphire laser after the propagation of its radiation through a tapered optical fibre are described. It is shown that the stability of the intermode frequency is impaired by half at short averaging times (10 s) and by a factor of 1.1 at long averaging times (1000 s). The results of investigations of the spectrum of a femtosecond Ti:sapphire laser, which is broadened in a tapered optical fibre are presented. It is shown that, by varying the fibre parameters and the characteristics of input pulses, the envelope of the broadened spectrum can be profiled, which is important for practical applications.

Multipoint fiber Bragg grating laser sensor interrogated by the intermodal beating frequency

We describe a technique for fiber Bragg grating multiplexing in a fiber Bragg grating laser sensor. This technique uses an intermodal beating frequency in a fiber laser with a linear cavity formed by a reference Bragg grating and several sensing Bragg gratings. We show the interrogation of an individual Bragg grating in an array formed by two, three, and four Bragg gratings. The longest cavity length used in the experiments was 8.67 km, with a 23-m distance separation between Bragg gratings, which proves the capability for long distance sensors with high spatial resolution. We consider that this technique is suitable for early warning and alarm systems.

Experimental investigations of the influence of a tapered fibre on the stability of the intermode frequency of highly stable femtosecond pulses

The experimental procedure and the results of high-precision measurements of the intermode frequency of a femtosecond Ti: sapphire laser at the output of a tapered fibre are described. In the process of mode transformation, the stability of the intermode frequency is shown to degrade roughly by a factor of 2 for short averaging time intervals (10 s) and by a factor of 1.1 for long averaging time intervals (1000 s).

Using geomagnetic birefringence to locate sources of impulsive, terrestrial VHF signals detected by satellites on orbit

The Earth's ionosphere is magnetized by the geomagnetic field and imposes birefringent modulation on VHF radio signals propagating through the ionosphere. Satellites viewing VHF emissions from terrestrial sources receive ordinary and extraordinary modes successively from each broadband pulse emitted by the source. The birefringent intermode frequency separation can be used to determine the value of ƒce cos β, where ƒce is the electron gyrofrequency and β is the angle between the wave vector k and the geomagnetic field B at the point where the VHF ray path intersects the ionosphere. Successive receptions of multiple signals (from the same source) cause variation in ƒce cos β, and from the resulting variation in the signal intermode frequency separation the source location on Earth can be inferred. We test the method with signals emitted by the Los Alamos Portable Pulser and received by the FORTE satellite.

High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser

A high-accuracy optical distance meter with a mode-locked femtosecond laser is proposed for distance measurements in a 310-m-long optical tunnel. We measured the phase shift of the optical beat component between longitudinal modes of a mode-locked laser. A high resolution of 50 microm at 240-m distance was obtained without cyclic error correction. The group refractive index of air is automatically extracted to an accuracy of 6 parts per million (ppm) by two-color measurement with the pulses of fundamental and second-harmonic wavelengths. Finally, an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency, together with the results of the two-color measurement.

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.

Stability and changes in pulse generation by actively mode-locked solid-state lasers

Theoretical and experimental investigations were made of various regimes of pulse generation in actively mode-locked solid-state lasers. These regimes were observed by altering the detuning of the modulation frequency of the intracavity losses from the intermode frequency of the cavity. The conditions for the appearance and the special features of operational instabilities, leading to changes in short pulse generation, were determined.