JP2910194B2 - Optical integrated pickup - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical pickup for reproducing information from an optical recording medium by using a laser beam, and more specifically, to an interference optical system configured using an optical waveguide. The present invention relates to an integrated optical pickup provided.

[Prior Art] Conventionally, in an optical recording medium in which information is recorded and reproduced by a laser beam, information is recorded by the length of a pit. In order to realize a high-density optical recording medium, it is necessary to record and reproduce information not only by the pit length but also by the pit depth. In such a high-density optical recording medium, an anti-vibration heterodyne interferometer using laser light interference is used to detect the depth of a pit. For example, an interferometer shown in FIG. 8 is one example.

Briefly, laser light emitted from a two-frequency orthogonal laser 500, such as a Zeeman laser, whose frequencies are composed of f _S and f _P and whose polarization planes are orthogonal to each other, is converted into a measurement beam and a monitor beam by a non-polarization beam splitter 510. 2
Minute. Monitor beam the difference frequency becomes the collated coherent light the polarization plane by the polarizer 570 (f _S -f _P) is monitored by an optical sensor 595, and the reference beat signal. On the other hand, the measurement beam reflected by the non-polarizing beam splitter 510 is expanded by a beam expander 520 and enters a bifocal lens 530 including an optical lens 531, a calcite 532, and an optical lens 533. The S wave whose polarization plane is perpendicular to the paper surface passes through the bifocal lens 530 without being stopped,
The light is focused on the surface of the optical recording medium to about 1 micron by the objective lens 540 and becomes a probe light. On the other hand, the P-wave whose polarization plane is parallel to the paper is focused on the back focal point of the objective lens 540 by the bifocal lens 530, becomes parallel light, and irradiates a wide area (about several hundred microns) of the optical recording medium and skid light. It becomes. The probe light having a sufficiently small spot diameter is subjected to a Doppler shift Δf when the optical recording medium having irregularities is driven by a driving element such as a motor. In addition, the Doppler shift ε due to the disturbance vibration also receives the light of the optical frequency f _s + Δf + ε, and follows the reverse path to the polarizer 58.
It enters the sensor 590 via 0. Also, the skid light, which is a P-wave, is sufficiently large as compared with the irregularities of the optical recording medium, and thus does not receive the Doppler shift due to the irregularities, receives only the Doppler shift ε due to the disturbance vibration, and becomes light having an optical frequency f _P + ε.
Following the reverse path, the polarization planes are aligned via the polarizer 580, and the multiplexed interference with the probe light is performed. As a result, the sensor 59
At 0, (f _S −f _P + Δf) from which the vibration component has been removed is measured. Displacement in the depth direction, by comparing the frequency fb = f _S -f _P of the previous reference beat light is the calculated by the conversion formula below seek Delta] f.

Δz = λ / 2 · ∫ ( Δf) · dt = λ / 4π · ∫ (δφ / δt) · dt = λ / 4π · (φ D -φ 0) ... (1) φ D is the phase measured, phi ₀ However, the anti-vibration heterodyne interference optical system is generally configured by arranging and fixing necessary optical components on a vibration isolator such as an optical bench. However, there is a problem that the optical axis is required to be finely adjusted, reliability and productivity are low, and the optical system is large and heavy.

The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to integrate a main part of an interference optical system on a single substrate using a thin-film waveguide technique. Another object of the present invention is to provide a compact, stable, and highly reliable optical integrated interference system that does not require optical axis adjustment.

[Means for Solving the Problems] In order to achieve this object, in the optical integrated pickup according to the first aspect of the present invention, a light source and a demultiplexer for guiding light from the light source and splitting the light into two optical waveguides. And an optical frequency shifter for giving a constant frequency shift to the guided light in one of the optical waveguides for guiding the light demultiplexed by the demultiplexing means, and one optical waveguide demultiplexed by the demultiplexing means. First optical means for converging and irradiating the light guided to the wave path to the pits of the recording medium to guide the reflected light, and light for guiding the light guided to another optical waveguide branched by the demultiplexing means to the first optical means. A second optical unit disposed in the vicinity of the first optical unit for irradiating the recording medium in a range larger than the first irradiating unit and guiding the reflected light, the first optical unit and the second optical unit; The reflected light respectively guided by the optical means An integrated optical interferometer comprising: a multiplexing interference unit for causing interference; and a sensor for guiding reflected light interfered by the multiplexing interference unit and detecting an interference signal of the reflected light.

In the optical integrated pickup according to the second aspect of the present invention, in addition to the configuration of the optical integrated pickup according to the first aspect, the optical integrated pickup is provided between the light source and the demultiplexing unit, and further transmits light from the light source to another. A second demultiplexing unit for demultiplexing the light into two optical waveguides, and a third demultiplexing unit that further demultiplexes the light demultiplexed and guided by the second demultiplexing unit to the two optical waveguides.
In one of the optical waveguides for guiding the light demultiplexed to the third demultiplexing means and the fourth demultiplexing means and the fourth demultiplexing means. A second optical frequency shifter and a third optical frequency shifter for giving a constant frequency shift to the reflected light, and a second optical frequency shifter and a third optical frequency shifter. A third optical unit and a fourth optical unit for converging and irradiating any one of the incident lights on the opposite edge portions in the track of the recording medium to guide the reflected light, and the third optical unit Alternatively, a second optical waveguide that couples light guided from each optical waveguide for guiding light to the fourth optical means and an optical waveguide that does not guide light to the third optical means or the fourth optical means. A multiplexing interference unit and a third multiplexing interference unit; Each of the lights multiplexed and interfered by the second multiplexing interference means and the third multiplexing interference means is guided, and a second sensor and a third sensor which respectively detect the interference signal of the reflected light are connected. A second integrated optical interferometer and a third integrated optical interferometer.

[Operation] In the optical integrated pickup of the present invention having the above configuration, the light emitted from the laser as the light source is split into three channels, and the first and third channel integrated optical interferometers for tracking control are recorded. It is led to the second channel interferometer for reading. Specifically, the light guided to the first and third channel integrated optical interferometers is further guided to three first, second, and third optical waveguides. The light guided to the second waveguide is given a certain frequency shift by the optical frequency shifter, and becomes f ₀ + f _R. The light having undergone the frequency shift is further split into two and guided to the fourth and fifth optical waveguides. First
The light guided to the waveguide is focused and irradiated on the edge of the track of the optical recording medium by the gradient index lens, and the reflected light is guided to the sixth waveguide. The light passing through the third optical waveguide and the light passing through the fourth optical waveguide are multiplexed and interfered by the multiplexer, and the interference signal is detected by the first optical sensor. The light passing through the fifth optical waveguide and the light passing through the sixth optical waveguide are multiplexed and interfered by a multiplexer, and the interference signal is detected by a second optical sensor. The tracking control signal is determined by the phase difference between the first and second signals of each channel.

Similarly, in the second channel interferometer, the light split into the channel is split into three and guided to the first, second, and third optical waveguides. The light guided to the second waveguide is given a certain frequency shift by the optical frequency shifter, and becomes f ₀ + f _R. The light having undergone the frequency shift is further split into two and guided to the fourth and fifth optical waveguides. The light guided to the first waveguide is focused and irradiated on the recording portion of the optical recording medium, and the reflected light is guided to the sixth waveguide. The light passing through the third optical waveguide and the light passing through the fourth optical waveguide are multiplexed and interfered by the multiplexer, and the interference signal is detected by the first optical sensor. The light passing through the fifth optical waveguide is applied to a wide area of the recording medium by the gradient index lens, and the reflected light is guided to the seventh waveguide. The light guided to the seventh waveguide causes multiplex interference with light passing through the sixth optical waveguide, and the interference signal is detected by the second optical sensor. The phase difference between the first and second signals is obtained by the conversion equation of the above equation (1) to determine the pit or the pit depth.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, for example, an integrated optical interferometer is a GGG (Gd ₃ Ga
₅ O ₁₂₎ formed on the substrate 10, such as a _{Bi: YIG (Bi x Y 3} -x Fe 5 O
_{12), Bi: GdIG (Bi} x Gd 3-x Fe 5 O 12) magnetic thin film 15 having a magneto-optical effect, such as, a semiconductor laser 20, an optical isolator 3
0, focusing lens 40, photodetector N_9 such as Si photodiode
0 and N_91. Here, N represents N channels, and hereinafter all are abbreviated as N.

As shown in FIG. 2, the magnetic thin film 15 is provided with a ridge portion 21 whose thickness is larger than other portions, thereby forming a ridge waveguide. The laser light propagates along the ridge portion 21. The ridge portion 21 is manufactured using well-known photolithography. That is, a magnetic thin film 15 is formed on the substrate 10 by a thin film forming means such as a liquid phase growth method or a sputtering method, and a photoresist is applied thereon. Then, after patterning into a predetermined ridge waveguide shape, a portion other than the portion to be the ridge 21 is etched by a predetermined thickness by plasma etching, sputter etching, wet etching using hot phosphoric acid, or the like. Finally, the ridge portion 21 is formed by removing the photoresist.

The laser light emitted from the semiconductor laser 20 propagates through the optical isolator 30 that passes the light in only one direction,
And is incident on the ridge-type waveguide 21. At this time, the semiconductor laser 20 is mounted so that the laser light propagating through the ridge-type waveguide 21 becomes a TM mode in which the direction of the magnetic field oscillation is parallel to the thin film surface. The laser light that has propagated through the ridge-type waveguide 21 is extracted by Y branches Y1 and Y2,
The light propagates through 1, 2, and 3 waveguides, respectively, and enters each of the channels 50, 60, and 70 constituted by the heterodyne interferometer. N
The light entering the channel is further split into three as shown in FIGS. 5 (a) and 5 (b), and the first, second and third waveguides represented by N_1, N_2 and N_3 respectively. I have. In each channel, on the branch waveguide N_2, a buffer layer N_115 made of SiO ₂ or the like and an electrode N made of metal such as Al
_120 are provided to constitute a frequency shifter N_110.
By flowing a current from the oscillator 80 to this electrode N_120,
A magnetic field perpendicular to the laser light propagation direction and parallel to the thin film surface is applied to the branch waveguide N_2. This magnetic field magnetizes the branch waveguide N_2, and the phase shift due to the magneto-optical effect is T
Occurs for M mode. That is, assuming that the propagation constant of the TM mode when no magnetic field is applied is β ₀ and the propagation constant when a magnetic field is applied is β, β = β ₀ + γ (1) Here, γ is the amount of phase shift due to the magneto-optical effect. As shown in FIG. 1, z
Axis, taking the x-axis in the direction perpendicular to the thin film plane, gamma is given by _{γ = 2ω 0 ε 0 ∫Re [} ε xz e x * e z] dx (2). Where ω ₀ is the angular frequency of the laser beam, ε ₀
Is the vacuum permittivity, e _x and e _z are one of the non-diagonal elements of the dielectric tensor of the electric field x component and z component of the TM mode, epsilon _xz magnetic thin film 15, * the complex conjugate, Re is Represents the real part of a complex number. ε _xz has a relationship with the Faraday rotation coefficient θ _F and ε _xz = j (2n ₁ / k) θ _F (3). Where n ₁ is the refractive index of the magnetic thin film 15,
k is the wave number of the laser light in a vacuum, and the wavelength of the laser light is λ
Then, k = 2π / λ (4) Further, in a range where the magnetization of the magnetic thin film 15 is not saturated, the magnetization of the magnetic thin film 15 is applied magnetic field H, that is,
Since it is proportional to the current I flowing through the electrode N_120, if this proportionality constant is V, then θ _F = VI (5).

(3) Substituting expressions (2) _{where, γ = 2ω 0 ε 0 (} 2n 1 / k) θ F ∫Re [je x * e z] dx = Aθ
_F (6). Here, A is a constant.

When the length of the portion where the magnetic field is applied in the branch waveguide N_2 a 1, field of the TM mode which has received the phase _{shift, e x = E x sin [} ω 0 t + (β 0 + γ) 1] = E _x sin [ω ₀ t + Aθ _F 1 + β ₀ 1] (7) _{Here, θ F = ω R t /} (Al) (8) i.e., from (5), when an electric current is applied to the electrodes N_120 such that _{I = ω R t / (AlV} ) (9), e x = E _x sin [(ω ₀ + ω _R ) t + β1] (10), and the angular frequency of the laser light is from ω ₀ = 2πf ₀ to ω ₀ + ω.
_R = 2π (f ₀ + f _R ), that is, the frequency of the laser light is f
It shifted by f _R from _0, and f ₀ + f _R. However, the electrode N_1
As shown in FIG. 3, the current of the reference signal flowing through 20 has an amplitude of 2π /
(AlV), similar frequency shift can be obtained as a sawtooth wave of period 2π / ω _R. Thus, the frequency of the laser light having the frequency f ₀ incident on the branch waveguide N_2 is shifted by the frequency f _R by the frequency shifter, and the frequency becomes the reference light f ₀ + f _R.

The first and third channel interferometers 50 and 70 are as shown in FIG. 5 (a). In this interferometer, the laser light propagating through the branch waveguide N_1 is distributed by the gradient index lens N_80. The light is emitted to the edges 102 and 103 of the track 100 of the rotating optical recording medium. The laser light reflected from the edges 102 and 103 is again indexed refractive index lens with the averaged phase information of the track and groove.
Enter N_80. For example, if the pickup head is slightly displaced in the direction of the rotation center, most of the irradiation light (first channel) to the track portion hits the track portion, so that the phase of the reflected light is advanced from the beginning, and conversely, the phase of the reflected light is delayed from 103. . Assuming that the phase change due to this track shift is ΔT _N / Δt, the frequency of the reflected light is f ₀ + ΔT _N / Δt, and enters the gradient index lens N_80. The entered light returns to the waveguide N_1 again, and a part of the light enters the sixth waveguide N_6. On the other hand, in the frequency shifter N_110, the reference light having undergone the frequency shift by f _R is branched into two by the Y branch N_Y5, and the respective branch waveguides
The reference light propagating through N_4 and N_5 and propagating through the branch waveguide N_5 is the same as the signal light propagating through the branch waveguide N_6 and the Y branch N_Y.
It is multiplexed by 6 and exits the optical integrated pickup. The emitted light enters the optical sensor N_90 via the mirror N_200, and the interference signal f
_R + ΔT _N / Δt is detected.

Further, the reference light that has propagated through the branch waveguide N_4 and the laser light that has not undergone the frequency shift that has propagated through the branch waveguide N_3 are multiplexed and interfered by the Y branch N_Y7, and exit the optical integrated pickup, and pass through the mirror N_200. Enter the optical sensor N_91. Then, the Doppler shift ΔT _N / Δt is obtained by comparing the output signals of the optical sensors N_91 and N_90, and tracking control is performed so that ΔT1 and ΔT3 are always constant.

FIG. 5B shows the interferometer of the second channel. In this interferometer, the laser light propagating through the branch waveguide N_1 is applied to the recording pit portion 101 of the optical recording medium which is rotating by the gradient index lens N_80. Recording pit 1
The laser light reflected at 01 has a phase difference ΔP corresponding to the pit depth information and a disk vibration component ε, f ₀ + ΔP / Δ
The light becomes t + ε and enters the gradient index lens N_80 again. For example, even in a memory form in which the depth information corresponds to 1-byte information as shown in FIG. 7, it can be read by one access. The light that has entered the gradient index lens N_80 enters the waveguide N_1 again, and a part of the light enters the sixth waveguide N_6. On the other hand, the reference light that has been frequency-shifted by fR by the frequency shifter N_110 is split into two by the Y branch N_Y5, propagates through the branch waveguides N_4 and N_5, and the reference light that has propagated through the branch waveguide N_5 is refracted. The diameter is reduced to several tens of microns by the gradient index lens N_81, and is radiated to a wide area as skid light, becomes f ₀ + ε light with the vibration component ε of the disk, and enters the waveguide N_5 again via the gradient index lens. A part of the light enters the waveguide N_7, and the light that has passed through the previous waveguide N_6 is multiplexed and interfered by the Y-branch N_Y8, the vibration component is removed, and the light having the frequency of f _R + ΔP / Δt passes through the mirror N_200. The sensor enters N_90 and is detected. Further, the reference light propagated through the branch waveguide N_4 and the laser light that has not undergone the frequency shift propagated through the branch waveguide N_3 are multiplexed and interfered by the Y-branch N_Y7 as before, and the interference signal fR passes through the mirror N_200. Is detected by the optical sensor N_91. Then, the phase difference between the signals obtained by the optical sensor N_91 and the optical sensor N_90 is obtained, and is converted into information ΔZ in the depth direction according to the equation (1). To briefly explain the conversion method, the conversion means converts the phase difference into a digital quantity by pulse-shaping the interference signal and counting it with a high-frequency quartz clock of, for example, 100 MHz as shown in FIG. For example, a reference beat frequency of 100 KHz, a high-frequency quartz clock of 100 MHz, a count value of M,
Phases phi _D measured, when the initial phase and phi _0, [Delta] P the phase difference caused by pits becomes _{ΔP = φ D -φ 0 = 2π} · (M D -M 0) / 1000.

Also, in terms of the amount in the height direction by the above formula (1), and Δz = λ / 4π · (φ D -φ 0) = λ / 2000 · (M D -M 0).

These are calculated by the conversion means such as the computer means shown in FIG.

By the way, the laser light returning to the branch waveguide N_1 propagates through the ridge portion 21 and the focusing lens 40 and enters the optical isolator 30, but the optical isolator 30 is directed only in one direction from the semiconductor laser 20 to the focusing lens 40. Since the light is transmitted, the return light from the optical fiber N_17 cannot pass, and the semiconductor laser
Does not reach 20. Therefore, in the semiconductor laser 20, since no noise is generated due to the return light, measurement with good S / N can be performed.

As described above, one embodiment of the present invention has been described in detail with reference to the drawings. However, the present invention is not limited to this embodiment, and various modifications can be made.

That is, the materials of the substrate, the magnetic thin film, the buffer layer, and the electrode are not particularly limited. For example, sapphire, glass, etc. are used for the substrate, rare-earth semiconductors such as CdMnTe, paramagnetic materials such as Faraday rotating glass, TAG (Tb ₃ Al ₅ O ₁₂ ), etc. An oxide or a nitride such as ZnO or AlN may be used for the layer, and gold or the like may be used for the electrode.

Alternatively, a magnetic thin film may be used only for the frequency shifter portion, and a dielectric material having no magneto-optical effect may be used for other portions. Further, the shape of the electrode for applying a magnetic field to the frequency shifter is not limited. Furthermore, a magnetic field may be applied by providing an external electromagnet, coil, or the like.

The multi-channel optical integrated pickup may be modified as shown in FIG. This is because the multi-channel optical integrated pickup shown in FIG.
_4 and the sensor N_91 have been removed for simplicity. In this case, a part of the output of the driver driving the frequency shifter is used as a substitute for the reference beat signal, and the phase information is converted into the presence or absence of pits or the pit depth information by the same means as the above-mentioned conversion means. is there. In this case, the frequency shifter may be provided on the optical waveguide N_1 or may be provided on the optical waveguide N_6. Although various other modifications are conceivable, those skilled in the art can implement the present invention in various modes without departing from the gist of the present invention.

[Effects of the Invention] As is clear from the above description, according to the present invention, the main components of the interference optical system such as a laser beam splitter, a multiplexer, and a frequency shifter using the thin-film waveguide technology. Since the units are integrated on one substrate, optical axis adjustment is not required, and a compact, stable, and highly reliable optical integrated pickup can be manufactured with high productivity. In particular, the probe and skid light optics, which previously required a complicated and large optical system, which has been a bottleneck in miniaturization, are to be separated as much as possible within the practical range to make the optics extremely compact and simple. Therefore, there is an effect that the whole apparatus can be made compact and simple.

[Brief description of the drawings]

FIGS. 1 to 7 show an embodiment embodying the present invention. FIG. 1 is a block diagram of a multi-channel optical integrated interferometer according to an embodiment of the present invention. FIG. 3 is a detailed view of a ridge portion, FIG. 3 is a diagram showing a waveform of a current applied to an electrode of a frequency shifter, FIG. 4 is a block diagram for counting a phase difference by a high frequency clock, and FIG. FIG. 6 shows a specific example of an optical integrated interferometer of each channel. FIG. 6 shows a modification of the optical integrated pickup of the present invention. FIG. 7 shows a track of an optical disk having information in the depth direction. FIG. 8 is a sectional view, and FIG. 8 is a pickup using an optical system of a conventional heterodyne interferometer. 1, 2, 3 ... optical waveguide 10 ... substrate 15 ... magnetic thin film 20 ... semiconductor laser 21 ... ridge portion, ridge type waveguide 30 ... optical isolator 40 ... focusing lens 50 ... first channel light Integrated interferometer 60… Second channel optical interferometer 70… Third channel integrated interferometer 80… Oscillator 1_90, 1_91… Optical sensor 2_90, 2_91… Optical sensor 3_90, 3_91… Optical sensor 100… … Track of optical recording medium 101… Optical recording pit 102… Top track of optical recording medium 1_200… Mirror 2_200… Mirror 3_200 Mirror 500… Dual frequency orthogonal laser generator 510… Non-polarizing beam splitter 520 …… Beam expander 530 …… Bifocal lens 531,533 …… Optical glass 532 …… Calcite 540 …… Condenser lens 550 …… Measurement object 560 …… Piezoelectric actuator 570,580 …… Polarizer 590,595 …… Optical sensor 700 …… Reversed 710 High frequency quartz clock 720 AND device 730 Counter 740 Bus 750 Computer means Y1, Y2 Y branch N_Y5 Y branch N_Y6 Y branch N_Y7 Y branch N_Y8 Y Branch N_1 N-channel first waveguide N_2 N-channel second waveguide N_3 N-channel third waveguide N_4 N-channel fourth waveguide N_5 N channel N_6... N-channel sixth waveguide N_7... N-channel seventh waveguide N_80... N-channel refractive index distributed lens N_81... N-channel refractive index distributed lens N_90 N-channel optical sensor N_91 N-channel optical sensor N_110 N-channel frequency shifter N_115 N-channel buffer layer N_120 N-channel electrode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-240447 (JP, A) JP-A-61-207903 (JP, A) JP-A-59-216008 (JP, A) JP-A-59-216008 214706 (JP, A) JP-A-2-176512 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 7/09, 7/095 G11B 7/135

Claims (2)

(57) [Claims] 1. A light source, a demultiplexing unit that guides light from the light source and splits the light into two optical waveguides, and one of the optical waveguides that guides the light split by the splitting unit. An optical frequency shifter for giving a constant frequency shift to the reflected light; and a first light guide for condensing and irradiating light guided to one optical waveguide demultiplexed by the demultiplexing means onto a pit of a recording medium to guide reflected light. An optical means for irradiating the recording medium with light guided to another optical waveguide demultiplexed by the demultiplexing means in a range larger than that of the first irradiating means and guiding reflected light thereof; A second optical unit disposed in the vicinity of the optical unit; a multiplex interference unit configured to multiplex and interfere the reflected lights respectively guided by the first optical unit and the second optical unit; The reflected light interfered by the multiplexing interference means is guided, Optical integrated pickup which comprising the integrated optical interferometer having a sensor for detecting the interference signal Shako. 2. A light source provided between said light source and said demultiplexing means,
Second demultiplexing means for demultiplexing the light from the light source to two other optical waveguides; and demultiplexing the light demultiplexed and guided by the second demultiplexing means to two optical waveguides, respectively. One of the optical waveguides for guiding the light demultiplexed to the third demultiplexing means and the fourth demultiplexing means, and the third demultiplexing means and the fourth demultiplexing means. A second optical frequency shifter and a third optical frequency shifter for giving a constant frequency shift to the guided light in the waveguide, and a second optical frequency shifter that is demultiplexed by the third demultiplexer and the fourth demultiplexer. Third optical means and fourth optical means for converging and irradiating any one of the lights guided to the two optical waveguides to the opposite edge portions in the track of the recording medium to guide the reflected light; Guiding the light to the third optical means or the fourth optical means A second multiplexing interference unit and a third multiplexing unit that multiplex and interfere light guided from each of the optical waveguides and an optical waveguide that does not guide the light to the third optical unit or the fourth optical unit. An interference unit, a second sensor that guides each of the lights multiplexed and interfered by the second multiplexing interference unit and the third multiplexing interference unit, and a second sensor and a second sensor that respectively detect an interference signal of the reflected light. The optical integrated pickup according to claim 1, further comprising: a second integrated optical interferometer having a third sensor and a third integrated optical interferometer.