Wavelength-Tuning Mechanism [Katagiri et al., 1998b]

Wavelength-tunable lasers are achieved by simply inserting wavelength-tunable optical bandpass filters in the ring lasers. For use in lasers they are expected to have low-loss, high-resolution tenability in wide ranges and polarization independence. Although there are many candidates for such filters, few of them can satisfy all of the above requirements. An exception is the dielectric filter, to be introduced here.

The dielectric interference filter generally consists of a resonance layer half the wavelength in thickness sandwiched by multiple pairs of quarter-wavelength-thick layers with different indices of refraction (Figure 3.50), and transmit only the light that matches with the resonance condition that gives the transmission spectrum with a Lorentzian profile as

(3.20) Here, λc is the transmission center given by

(3.21) where h is the thickness of the resonance layer, n is its index of refraction, and ∆λ is the transmission bandwidth at 3 dB.

Directly changing the thickness of the resonance cavity may be reasonable for achieving wide tunability;

however, conventional methods of using thermal or mechanical expansion of dielectrics are completely inadequate for the above purpose. One of the most effective ways uses a wedged structure for the resonance layer (see Figure 3.51). The thickness of the resonance layer is effectively changed according to the beam position. Such a tuning mechanism is realized using a circularly wedged, disk-shaped optical bandpass filter and a rotary positioning system (see Figure 3.52) [Thelen, 1965; Apfel, 1965]. Digital marks are drawn on the fringe of the filter disk and are read by a sensor to produce encoded signals including conventional Z, A, and B signals (see Figure 3.53). These signals are detected and immediately analyzed by a programmable logic gate circuit to determine the absolute position and relative displacement of the disk. This information is processed by a CPU to control the ultrasonic motor as a rotary actuator.

FIGURE 3.50 Structure of optical bandpass filter using multiple dielectric layers.

2λn

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Both precise scanning and positioning are possible for the disk according to the preinstalled programs.

The optical system maintains vertical incidence throughout such tuning operations. This enables precise wavelength tuning performance over a wide range while maintaining constant total and low polarization-dependent losses [Hashimoto and Katagiri, 2001]. The precise wavelength tuning is based on the wave-length calibration using a relationship between the center wavewave-length and the digitized position (see Figure 3.54). Figure 3.55 shows an example of such a tuning mechanism for the disk-shaped filter.

FIGURE 3.51 Control of length of effective resonators formed in a collimated beam by light-beam positioning based on wedged-layer structures.

FIGURE 3.52 Disk-shaped wavelength-tunable optical bandpass filter. The filter disk has a circularly wedged cavity region and the center wavelength is determined by positioning the rotation angle of the disk. The position control uses an ultrasonic motor under the control with a high-resolution rotary encoder system.

FIGURE 3.53 Encoded signals for determining the absolute position on a disk.

effective resonators collimated beams

wedged resonance layer HRC

HRC

beam positioning

optical fiber collimated beam

mark(Z) mark (A, B) sensor

position detection CPU

drive signal

ultrasonic motor

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A laser with a wide wavelength-tuning range is constructed using a disk-shaped, wavelength-tunable bandpass filter. The intensity of the laser light generated is stabilized by an SOA under gain-saturated conditions. Applying this stabilization method to the ring laser with an optimal filter disk and SOA as a gain medium, wide tenability in the 1530- to 1590-nm range is achieved in the communication bands under sufficient intensity-stabilized operating conditions (see Figure 3.56). A wavelength-scanning laser, which is of great use for optical measurement, can also be realized by rotating the disk synchronously with an electrical signal. When the scanning speed is optimized so that laser oscillation instability is suppressed, this kind of synchronous laser-wavelength scanner is achieved, and it maintains a line width of below 1 GHz. Figure 3.57 shows a typical example of how to measure the spectral response of an acetylene gas cell that has been used for absolute-wavelength calibration.

3.4 Conclusions

This chapter describes the principles of semiconductor lasers and their applications. Because semicon-ductor lasers are extremely small light sources that can produce an almost coherent light, they have proven useful for a wide variety of practical applications.

FIGURE 3.54 Tunable performance of a disk-shaped optical bandpass filter.

FIGURE 3.55 Mechanism of a disk-shaped wavelength-tunable filter.

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Semiconductor lasers are now evolving toward future high-performance applications, as shown in Figure 3.58. For example, ultra-narrow-linewidth semiconductor lasers realized by reducing the phase noise are now used for detecting gravitons in geophysics. By reducing the noise even further, we can realize a pure quantum-mechanical state of light. Such light will be useful for quantum-mechanical computing FIGURE 3.56 Tuning range of a wavelength-tunable ring laser.

FIGURE 3.57 Application of wavelength-tunable laser to optical measurement. The laser is used in the scanning mode. The spectrum shows the absorption lines of C₂H₆ gas.

FIGURE 3.58 Future prospects of high-performance semiconductor lasers.

years functionality

2005 2010 2015 2020

photonic network

quantum computing

noise reduction PLL stabilization hyper-coherent laser

coherent control

wavelength tuning

MEMS technology nano-technology

CQED

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and communications. We can also produce ultra-high-frequency electromagnetic waves in the millimeter-frequency range as another attractive implementation of semiconductor lasers that is based on strict control over wavelength and phase of the lasers. Although conventional IR-sensing applications have been satis-factory sources for this, novel applications are being developed in communication-related fields.

Consequently, we all accept the fact that semiconductor lasers are very attractive because their laser oscillation performances are readily controlled by a wide variety of schemes using physical and mechanical techniques. The future no doubt promises further development in the area of semiconductor lasers supported by novel nano-technology as applied to the field of microscopic material processing and positioning technologies [Chang and Campillo, 1996].

Defining Terms

antireflection coating: Conventional semiconductor lasers have a facet reflectivity of 32%, which comes from the refractive index of semiconductor media of around 3.5. Antireflection (AR) coatings are used to reduce laser-facet reflectitivity with a residual reflectivity of less than 0.1%. The AR coatings consist of single or multiple dielectiric layers. An AR coating with a single layer has a thickness of a quarter wavelength and an optimum refractive index equal to the square root of the media. The optimum value for the laser media is therefore around 1.7, taking into account the above index.

Materials suitable for AR coatings for laser media include glass films such as SiO2 (refractive index rf = 1.54), Si3N4(rf = 1.98) or their compound. An AR coating with multiple layers consisting of two kinds of films with different indices can reduce the laser-facet reflectivity in a wide wavelength range. AR coatings are used to construct external-cavity lasers in which the light circulates, passing through this kind of AR-coated facet.

multisegment lasers: Semiconductor lasers can be divided into independent segments, each of which is electrically isolated by the formation of etched grooves. Multisegment lasers consist of these isolated segments. When the depth of the grooves is optimized, electrical isolation is achieved while lightwaves traveling along laser waveguides feel negligible effects of the grooves. The grooves are usually several micrometers deep and around 20 mm wide.

quantum well: Electrons are electrically confined by potential barriers. Using two planar barriers, elec-trons are two-dimensionally confined. This kind of confinement structure exhibits a well shape.

When the spacing of the two barriers becomes extremely small, the electrons confined in the well exhibit wave-like properties in accordance with quantum mechanics. The distinguishing feature of electrons in these quantum wells is their discrete energy levels. This discreteness facilitates light emission at a particular wavelength corresponding to an appropriate energy-level difference; hence it improves the laser oscillation performance.

recombination: When a light is absorbed in laser media, pairs of an electron and hall are produced.

Although they may remain in the media, they are usually coupled together quickly and emit light.

Such coupling is called recombination.

relaxation process: In semiconductor media, electrons are excited from a ground level to upper-energy levels by absorbing a light power. These excited electrons may return to the ground level by emitting excess energy corresponding to the light power. This kind of energy-level transition is called relaxation. Generally, relaxations have different processes. One is a photon-emission process, which means light emission, while the other is an electron-emission process in which an electron carries away the energy as kinetic energy. The former process is useful for laser oscillation, while the latter process is undesirable.

saturable absorber: Semiconductor laser media act as lightwave absorbers by applying a reverse-bias voltage to the media. While the absorption coefficient remains constant for the lower input light power, it is dramatically reduced when the input light power increases above an appropriate power level, depending on media materials. Due to such saturation the semicondcutor laser media are called saturable absorbers. A typical feature of the saturable absorbers is pulse-width narrowing.

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When optical short pulses having a temporal profile with pedestals pass through such saturable absorbers, the pedestal portion with its lower power is whittled away, while their steeple-top portion with its higher power remains. Consequently, the pulses are narrowed.

ultrasonic motor: Ultrasonic motors are formed with arrayed vibrators that cooperatively move to generate a traveling wave. Objects attached to these vibrators are driven along the direction of the traveling wave by friction forces produced between the object and vibrator surfaces. Linear or circular drives are possible depending on configurations of the vibrators. The vibrators are typically made from piezoelectric thin films and the vibration frequency is in the several-megahertz range, equivalent to the ultrasonic frequency range. These ultrasonic motors are remarkable for their small size and high-torque performances and hence are suitable for opto-mechatronic applications.

References

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4.2 Optical Sensors for Displacement Sensing

4.3 Basic Principles and Methodologies

4.4 Novel Applications to Metrological Sensing

New Position Sensors for Straightness Measurements

• Optical Encoder with Pitch-Modulated Photodiode Array • Integrated Grating-Image-Type Encoder • Integrated Interferometric Sensors

4.5 Conclusions

4.1 Introduction

Optical sensors are indispensable for the precise control of mechatronic systems as well as other electronic and mechanical sensors. Optical sensors are especially preferable in noncontact measurements because light transmission and reflection can be used without contacting the object surface. Moreover, measurements using light as a means of detection are essentially the fastest in response because of their inherent propagation velocity, although the response time is limited by the detection electronics. In the case of displacement sensing using light waves, optical sensors are essentially high precision as the wavelength can be utilized as a unit length for the measurement, which is in the submicron region of visible light.

Several optical sensing techniques have been proposed for mechanical applications in industry.

Table 4.1 shows the optical sensing technique used for mechatronics. They are categorized into several groups based on the sensing principle. The basic techniques consist of using the intensity of reflected light, straightness of light propagation, and interference of superimposed light beams. The position of the object can be detected based on the intensity of light reflected from the object. A high sensitivity can be obtained within a short range by using light focused with a microscopic objective. Based on the straightness of a light beam, the deviation from the optical axis can be sensed with simple semiconductor position sensors.

The highest precision is generally obtained using the optical interference technique because the wave-length of the light is used as the standard of wave-length. With linear displacement measurement along a laser beam axis, a laser with high temporal coherence is used. The signal processing technique—sophisticated for fringe interpretation—has already been developed using microcomputers. The Fourier transform method and phase-shift technique are powerful tools for processing interference images.

With the time-of-flight technique, the distance between the light source and the target is directly measured from the round-trip time of the pulsed light. Thanks to the development of high-frequency electronics, measurement accuracy was improved recently to less than 1 cm in commercial distance meters.

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Dans le document OPTO-MECHATRONIC SYSTEMS HANDBOOK (Page 110-117)