JPH0372227A - Method and apparatus for determining wavelength of optical radiation - Google Patents

Abstract

PURPOSE: To device a wavelength of an optical radiant ray by providing multiple wavelength dependent phase modulators which change the radiant ray together with the wavelength and have net phase modulation effect which becomes zero at a specified wavelength. CONSTITUTION: A phase modulator 1 contains a base layer 2 of niobic acid- lithium acid-lithium, and then in it, a single mode waveguide 3 is provided, and a silicon layer 4 is provided on it, and further two electrodes 5 and 6 are provided on it. When a voltage is applied across the electrodes 5 and 6, an electric field is formed, so that a refraction factor of the waveguide 3 is changed. Any light passing through the wavelength is subject to phase shift according to the applied voltage. Two modulators 11 and 12 which are provided to a wavemeter 10 and whose electrode structures are different from each other are so connected in series that phase shift conversion applied by voltages V1 and V2 are offset each other at a specified wavelength. A specified wavelength is offset by applying the same voltage to both electrodes and selecting relative lengths of electrodes 13-16, or applying different voltages to the modulators 11 and 12.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for determining the wavelength of optical radiation, such as visible light, ie a wavelength meter.

There are many cases in which it is necessary to measure, stabilize or control the wavelength of optical radiation. Common measurement methods include using a monochromator or determining wavelength from measurements of the temperature of the source. However, in some cases, the equipment required for such methods is typically bulky, expensive, or of insufficient precision for practical use. This is particularly true for fiber optic sensors and communication VT systems.

An example of where the use of such a wavelength meter is useful is in fiber optic gyroscopes. The gyroscope's scale factor (ie, the measured rotational speed relative to the applied rotational speed) is wavelength dependent. The problem is that the wavelength of light from some light sources, such as edge-emitting light-emitting diodes (ELEDS), which are very common for use in fiber optic gyroscopes, changes with temperature, causing the fiber optic gyroscope's scale elements to significantly dependent on temperature,
That's true.

A first object of the invention is to provide another type of wavelength meter that is suitable for incorporation into a fiber optic gyroscope and that produces an output that is used to stabilize the wavelength and/or modify the scale elements of the gyro. It is to provide.

Another object of the invention is to provide a wavelength meter that can be manufactured using composite optical technology.

A further third object can be implemented as an additional function in a composite optical device for use in fiber optic gyroscopes, retaining its primary function, namely the phase modulation of the light passing through the device, thereby improving said additional function. l! It is an object of the present invention to provide a wavelength meter that can be used with relatively little additional cost.

According to a first feature of the invention, there is provided a method for determining the wavelength of optical radiation, said method comprising the steps of: determining the wavelength of optical radiation; The method is provided, comprising determining a net phase modulation of radiation through a wavelength dependent phase modulator and then determining a difference between the wavelength and the constant wavelength.

According to a second feature of the invention, there is provided a device for determining the wavelength of optical radiation, wherein the device causes the optical radiation to have a net phase modulation effect that varies with wavelength and is zero at a constant wavelength. There is provided a device comprising: a phase modulator subjected to a plurality of wavelength-dependent phase modulations having a wavelength-dependent phase modulation of the radiation;

According to a third feature of the invention, a wavelength-controllable optical radiation source is provided, which modulates the radiation from the source with a net phase modulation effect that varies with the wavelength of the radiation and is zero at a constant wavelength of the radiation. Phase shift through multiple wavelength-dependent phase modulators with 1
An optical device is provided that includes a liJ device and a wavelength adjustment device that responds to changes in the net phase modulation effected by the radiation and correspondingly controls the wavelength of the radiation produced by the radiation source.

According to a fourth feature of the invention, there is provided an optical device for measuring a certain physical parameter, the device being provided with an optical radiation which is dependent on the physical parameter.
an optical transducer device that generates a characteristic of one line; a measuring device that receives said radiation from a transducer and measures said characteristic in order to determine said parameter; receiving optical radiation from a phase modulating device through a number of wavelength-dependent phase modulators having a net phase modulating effect of zero at a constant wavelength of , and determining the net phase modulation effected by the radiation; Irrespective of said determination, said optical device is provided with a phase sensing device which supplies said measurement device with information regarding the wavelength of the radiation.

According to a fifth feature of the invention, in use a base layer having a waveguide passing through it for transmitting light, the waveguide having two chain phases arranged parallel to each other and supported by said base layer. a modulator, each of the modulators having a pair of substantially parallel elongated electrodes so as to impart significantly different phase differences to the light when, in use, voltages are applied to the multiple pairs of electrodes; For each modulator, an electric field is created that overlaps the field of light passing through the waveguide, and the size of the modulator and each voltage applied to it produces a net phase shift for light of a certain wavelength. A wavelength meter is provided in which the wavelength is determined to be not.

Hereinafter, an explanation will be given based on the drawings as an example.

In FIG. 1, the phase modulator, generally designated 1, has a base layer 2 of lithium niobate, in which a single mode waveguide 3 is provided. A silicon layer 4 is provided on top of the waveguide, on which two electrodes 5.6 are provided.

When a voltage is applied across the electrodes, an electric field is created which changes the refractive index of the waveguide 3. Any light passing through the waveguide undergoes a phase shift according to the applied voltage. The magnitude of this phase shift depends on the length of the waveguide, the electro-optic constants, the refractive index of the material, the wavelength and the structure of the electrodes.

In FIG. 2, the wavelength meter 10 according to the invention has two phase modulators 11, 12 as in FIG. Two modulators with different electrode structures are connected in series to generate voltages VI and Vz.
are connected such that the phase shift transformations applied by cancel each other out at specific wavelengths. Cancellation for a particular wavelength can be achieved by applying the same voltage (i.e. V I = V Z ) to both electrodes and choosing the relative lengths of the electrodes 1.3.14i 15,16 or by using different This can be accomplished by applying a voltage to each modulator or by a combination of both.

The phase shift introduced by each modulator 11 or 12 depends, respectively, on the overlap of the electric fields between the electrodes 13, 14; 15, 16 and on the mode of the optical field traveling along the waveguide. If the electric fields of the modulators 11, 12 are different, they overlap the waveguide with different optical modes. However, 1
The other modulator needs to be longer, or the voltage on one electrode needs to be increased alternately to make the two phase modulators cancel each other out. However, the phase shift depends on the mode shape of the light traveling along the waveguide. The mode shape depends on the waveguide.

Thus, if the variations in wavelength are different for each modulator, cancellation is arranged to occur at only one wavelength, as shown in FIG.

-S-wise, the phase of the modulator can be changed by changing any combination of the following parameters:

fat the position of the waveguide relative to the electrode, fb) the length of the electrode of each modulator pair, (the voltage applied to the C1 electrode,
and/or fdl waveguide structure.

Different modulator structures are shown in FIG.

The modulators 17, 18 are different due to the different overlap of the electric fields with respect to the guided mode of the waveguide. This is modulator 1
7 differs from modulator 18 in that a stronger overlap occurs. The structure of the modulator 19 is such that the two outer electrodes are connected together and the signal is applied to the inner electrode. Any two of these modulators are connected together in series and used as a wavelength meter due to their different phase shift versus wavelength characteristics. These examples apply to Z-cut lithium niobate composite optics. However, the same theory can be applied to other types of composite optics made from X-cut lithium niobate, gallium arsenide or indium phosphate, for example.

The above description is approximate since the mode shape changes with wavelength, and the wavelength meter responds to changes in mode shape but not directly to wavelength. Nevertheless, the relationship between mode shape and wavelength is strong enough to make a wavelength meter. Changes to temperature are systematic and can be compensated for by measuring the temperature of the composite optical device.

In fact, the device in which the wavelength meter is to be used is capable of measuring phase shifts characterized by wavelength changes. The device may include an interferometer or other device, such device including an interferometer and a polarimeter or other device, an interferometer or polarimeter in which the wavelength meter is provided. The zero phase shift that can be added to is preferably modulated, for example by a sinusoidal waveform.

By way of example only, a possible application to a wavelength meter is a fiber optic gyroscope, one type of which is shown in FIG. Here, the light passes from the light a20 to the fiber coupler 21,
Then to a polarimeter 22 and a further optical power splitter 23
(coupler or Y-junction) and then to several phase modulators 24, 25, 26 and 27. The light is then transmitted to both ends of fiber optic coil 28. The returned light passes back through the modulator, optical power splitter, polarization splitter and coupler and reaches the detector 29. Such an arrangement is the auxiliary optical device of the fiber optic gyroscope Sagnac interferometer. The composite optical device includes one or more modulators and couplers, a combination of polarization demultiplexers or, in the case of m-v semiconductors, a composite optical device light source and detector.

Signal modulator 30 energizes the electrodes of modulator 34 on a composite optical chip, generally designated 31. The purpose of the signal is to determine whether there is a non-reciprocal phase shift between the two waveforms passing around the coil. Possible waveforms are
When T is the time delay around the coil, the amplitude ±π/4
, is a square wave with a frequency of 27T. If a non-reciprocal shift occurs, it can be detected by synchronously demodulating the rate demodulator 32 waveform. The output 33 of the rate demodulator is combined by a loop filter 34 to energize a Serrodyne voltage controlled oscillator (VCO) 35 whose output 36 eliminates the non-reciprocity caused by the rate demodulator. The output of the VCO is communicated to modulator 25 to eliminate the non-reciprocity provided by the rate demodulator. Seredine VC
The output of O is a sawtooth wave with an ideal peak of peak amplitude 2 and π, where m is an integer. The amplitude is controlled by a peak phase demodulator 37, which is energized by an error pulse that occurs synchronously with the resetting of the serrodyne waveform. The error is seven rhodyne amplitudes at 22mπ■
It is the output of the peak phase loop filter 38 to control CO.

Such fiber optic gyroscopes are common devices requiring accurate wavelength measurements, for which the proposed wavelength meter is suitable.

The wavemeter signal generator 39 outputs two sinusoidal signals 40.degree. 41 respectively to the two wavemeter modulators 26, 27 in such a way that the net modulation is not detected by the detector. The modulation frequency is chosen to be higher than the closed loop response of the rate loop and shorter than the handwidth of the gyroscope's rate demodulator.

Wavelength demodulator 43 demodulates signal 33 into signal 40 . Composite signal 44 is zeroed when link 45 is connected.

The rate demodulated output signal, which is demodulated synchronously with the wavelength meter signal generator, is the magnitude of the wavelength error of the device. The output can be used in various ways.

For example, a microprocessor (not shown) may observe and modify the scale elements. Otherwise, it can be used as a control signal for a wavelength control device (such as a Bertier) in a light source (also not shown). In another example, the voltage ratio of the signal sent to the wavelength meter is controlled in a closed loop to obtain a new zero. The voltage ratio can be read by a microprocessor modified scale element.

The fiber gyro device shown in FIG. relatively high (e.g. 0, l and 0
．． (between 9 times).

The wavelength meter signal generator, when used in the apparatus shown in FIG. 5, can use pseudorandom signals. The advantage of this is that it helps avoid disparate loops forming vehicle harmonics or harmonic lock-in.

The fiber gyro device shown in FIG. 5 has a wavelength meter associated with one arm of the Sagnac interferometer, but the modulator (signal and wavelength meter) is connected to a bush pull device (i.e. each half is attached to one of the composite optical devices). In addition, the wavelength meter can be used as well in a Matsuha-Tourunda interferometer (or other types of interferometers or polarimeters), and can be used on either the light source or the combination of the two devices. It can be placed in one of the waveguides between the detector and the Sagnac interferometer, or it can be installed entirely separately from the Sagnac interferometer.The later structure uses light from the bright leg of the coupler that is closer to the light source. half of the light source output is absorbed and lost to the device.

In yet another application, the wavelength meter used as described above comprises a coherent communication device or an optical sensor.

The wavelength to be measured is determined either before or after construction of the device.

The relative position of the modulators is not important. Any 11 alignment is not important. Any combination of scales may be used to implement the invention.

Additionally, electrical or optical bush-pull devices are commonly used in wavelength meters.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a phase modulator made of a Z-cut lithium niohate crystal, Figure 2 is a diagram of a wavelength meter including two phase modulators as shown in Figure 1, and Figure 3 is a cross-sectional view of a phase modulator made of a Z-cut lithium niohate crystal. Figure 4 is a cross-sectional view of several different phase modulators; Figure 5 is a block diagram of a fiber optic gyroscope containing the wavelength meter of Figure 2. figure. I - phase modulator, 2 - base layer, 3 - waveguide, 4 silicon layer, 5.6 - electrode, 10 - wavelength meter, 11° 12 - phase modulator, 13, 14, 15. 16.・-・Electrode, 17.1
8-phase modulator, 20-...-light source. 2 ] ° fiber coupler, 22 - polarizer, 23 - power splitter, 24. 25 - phase modulator, 26 ° 27 - wavelength meter modulator, 28 - optical fiber coil. 29 - detector, 3 (1 - signal modulator, 31 - composite chip, 32 - rate demodulator, 34 - modulator 35 - voltage controlled oscillator, 3 fl - loop filter.

Claims (1)

Claims: 1. A method for determining the wavelength of optical radiation, the method comprising: determining the wavelength of optical radiation, the method comprising: determining the wavelength of the radiation; The method includes determining a net phase modulation of the radiation through a phase modulator and then determining the difference between the wavelength and the constant wavelength. 2. An apparatus for determining the wavelength of optical radiation, wherein said apparatus subjects the optical radiation to a number of wavelength-dependent phase modulations having a net phase modulation effect that varies with wavelength and is zero at a constant wavelength. said apparatus, further comprising a phase measuring device for determining the net phase modulation effected by the radiation. 3. An optical radiation source with controllable wavelength, which transforms the radiation from the source into a number of wavelength-dependent phase modulators that have a net phase modulation effect that varies with the wavelength of the radiation and is zero at a constant wavelength of the radiation. an optical device comprising a phase modulation device for passing through and a wavelength adjustment device responsive to changes in the net phase modulation effected by the radiation source to correspondingly control the wavelength of the radiation produced by the radiation source; . 4. The method of claim 1, wherein the measured wavelength difference can be a function of temperature which can be measured and compensated for. 5. An optical device for measuring a certain physical parameter, the device being an optical transducer device which receives optical radiation and produces a characteristic of the radiation that depends on the physical parameter; a measurement device for measuring said properties in order to determine said parameters; a phase modulating device through a dependent phase modulator, and receiving optical radiation from the phase modulating device, determining the net phase modulation effected by the radiation, and regardless of said determination, transmitting the radiation to said measuring device. Said optical device comprising a phase sensing device providing information regarding wavelength. 6. In use, a base layer having a waveguide therethrough for transmitting light, two chained phase modulators disposed substantially parallel to the waveguide and supported by the base layer; each has a pair of long electrodes extending substantially parallel to each other,
In use, when a voltage is applied to each pair of said electrodes, an electric field is created for each modulator that overlaps with the field of light passing through said waveguide, so as to impart significantly different phase differences to said light; A wavelength meter in which the size of the modulator and each voltage applied to it are determined such that there is no net phase shift for light of a certain wavelength.