WO1995023993A1

WO1995023993A1 - A multielectrode, tunable optical filter

Info

Publication number: WO1995023993A1
Application number: PCT/US1995/002687
Authority: WO
Inventors: Jing-Jong Pan; Zhimin Liu; Qing-Yu Li; Yonglin Huang
Original assignee: E-Tek Dynamics, Inc.
Priority date: 1994-03-03
Filing date: 1995-03-02
Publication date: 1995-09-08

Abstract

A tunable optical filter of the Fabry-Perot interferometry type is provided. The filter has a pair of mirrors (12, 13) with reflecting flat faces facing and substantially parallel to each other. A tubular piezoelectric transducer (20) with a longitudinal axis (24) perpendicular to the mirror faces has an end attached to one of said mirrors (12, 13). A first electrode (22) is attached to the inner or outer surface of the piezoelectric transducer (20) and at least three, preferably four, second electrodes (23) are attached to the other surface of the transducer (2) so that the piezoelectric transducer (20) is located between the first electrode (22) and the second electrodes (23). Each second electrode (23) extends parallel to the longitudinal axis (24) and is spaced apart from each other in a plane perpendicular to the longitudinal axis (24). The faces of the first and second mirrors (12, 13) may be moved with respect to each other to maintain the mirror faces parallel to each other and to tune the filter by voltage signals.

Description

A MULTIELECTRODE, TUNABLE OPTICAL FILTER

BACKGROUND OF THE INVENTION

The present invention is related to the field of tunable optical filters and, more particularly, to tunable optical filters using Fabry-Perot interferometers. Tunable optical filters are devices which can receive light having many different frequencies as input and select light only of a given frequency as output. Such filters have many applications, such as in optical fiber networks. In a optical fiber network using wavelength division multiplexing, for example, tunable optical filters are invaluable in ensuring that a particular light signal travels from one source to the desired destination.

Generally speaking, a tunable optical filter of the Fabry-Perot interferometer type is centered about a pair of parallel mirrors. Light from an input fiber is sent into the cavity between the two mirrors and received by an output fiber. From the interference of the light in the cavity, the spacing between the parallel mirrors "tunes" the filter so that only the light having the resonant frequency with respect to the mirror spacing is transmitted as output.

The Fabry-Perot interferometer must have the mirrors parallel to each other. Deviation from perfect parallelism causes the transmission peaks of the output light signal to be lowered, the passband, i.e., the spectral width, becomes greater, and the central wavelength of the passband is shifted. Stated differently, the transmission loss is increased, the selectivity of the filter is lowered and the selected wavelength is changed. These are undesirable results for an optical filter.

This sensitivity to any slight deviation of the mirrors from parallel makes the manufacture of this type of filter difficult. The creation of mechanical jigs with the required high precision is difficult and expensive. Furthermore, such jigs must be stable for the period of the assembly of the filter. Environmental influences, such as air flow, mechanical vibrations and temperature fluctuations, not only affect the assembly of the filter, but also affect the filter once it is assembled. One approach has been the use of piezoelectric transducers. In one high precision Fabry-Perot interferometer three mechanical tilt adjustment stages are used. An independent piezoelectric stack or rod is mounted on each of these stages. Coarse adjustments of the mirrors are performed by the mechanical stages. Individual control bias voltages applied on each of the piezoelectric rods make the fine adjustments of the mirror positions. However, this type of filter is bulky and expensive, and is not suitable for emerging fiber optic networks in which miniaturization and low cost are very important.

The present invention uses piezoelectric transducers also, but offers a tunable optical filter of the Fabry-Perot type which is easily miniaturized and low cost.

SUMMARY OF THE INVENTION

The present invention provides for a tunable optical filter of the Fabry-Perot interfero etry type. The filter has a pair of mirrors with reflecting flat faces facing and substantially parallel to each other. A tubular piezoelectric transducer with a longitudinal axis perpendicular to the mirror faces has an end attached to one of said mirrors. A first electrode is attached to the inner or outer surface of the piezoelectric transducer and at least three, preferably four, second electrodes are attached on the other surface of the transducer so that the piezoelectric transducer is located between the first electrode and the second electrodes. Each second electrode extends parallel to the longitudinal axis and is spaced apart from each other in a plane perpendicular to the longitudinal axis. The faces of the first and second mirrors may be moved with respect to each other to maintain the mirror faces parallel to each other and to tune the filter by voltage signals between the first electrode and the second electrodes.

The present invention allows for the mirror faces to be critically aligned not only during the assembly of the tunable filter, but also after filter is installed and is subject to enviromental stresses.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a tunable optical filter according to one embodiment of the present invention;

Fig. 2 is a perspective view of the tunable optical filter of Fig. 1;

Fig. 3A is a perspective view of the piezoelectric transducer assembly in Figs. 1 and 2; Fig. 3B is a perspective view of the piezoelectric transducer assembly in Fig. 3A; and

Fig. 3C is a cross-sectional view of the piezoelectric transducer assembly in Fig. 3A;

Fig. 4 is a block diagram of the controller circuit for the piezoelectric transducer assembly; Fig. 5A is a detailed perspective view of the piezoelectric transducer assembly holder in Figs. 1 and 2; Fig. 5B is a detailed perspective view of the mirror holder in Figs. 1 and 2 ; and

Fig. 6 is a cross-sectional view of the fiber/collimator assembly for each one of the input and output fibers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) Some numerical results may be useful in illustrating the criticality of mirror alignment in an tunable optical filter of the Fabry-Perot interferometer type. For 3mm X 3mm mirrors, deviations of less than a micron between the separations at opposite edges of the mirrors results in a significant degradation in the finesse of the filter. A 0.0001 radian of tilt between the mirrors, i.e., an approximately 0.3 urn deviation, almost doubles the bandwidth of the filter (reduces the finesse by one-half) , lowers the transmission peak by one- quarter, and permits sidelobes to appear about the peak.

As noted above, the required tolerances for operable filters are achieved at high complexity and cost. Jigs for mechanical assembly typically have a tilt resolution of 10^"4 radian, with 10^"5 radian a possibility by chance.

The present invention offers a simple, and relatively low cost, filter with performance to match, or exceed, present filters. Fig. 1 shows a side view of one embodiment of the present invention. The filter illustrated in Fig. 1 has a base 10 on which is mounted a tube assembly holder 11 for holding one end of a piezoelectric tube assembly 20. A mirror 13 is mounted at the other end of the assembly 12. A second mirror 14 faces the first mirror 13 and is held in place by a mirror holder 15 mounted to the base 10.

As shown in Fig. 1 and the perspective view of Fig. 2, an end of an input fiber 16 is mounted to the tube assembly holder 11. An end of an output fiber 17 is mounted to the mirror holder 15. Not shown in the drawing are collimators, which are placed before the end of input fiber 16 and output fiber 17 so that light from the end of the input fiber 16 is collimated, sent through the hollow piezoelectric tube assembly 20 to the mirror 13. The light then passes through the mirror 13 to the mirror 14 and is reflected between the cavity formed between the front surfaces of the mirrors 13 and 14. The light at the resonant frequency determined by cavity passes through the mirror 14, and is refocused by the collimator in front of the output fiber 17 on the output fiber end. The piezoelectric tube assembly 20 ensures the critical alignment of the mirrors 13 and 14. Fig. 3A-3C illustrate assembly 20 in detail. The assembly is formed from a tube 21 of piezoelectric material, approximately 0.65 inches long with a diameter of 6mm and a wall thickness of 0.020 inches. A tubular electrode 22 fits on the inside of the piezoelectric tube 21. - 5 -

On the outside of the tube 21 are attached four electrodes 23 which extend parallel to the longitudinal axis 24 length of the piezoelectric tube 21. As illustrated in Fig. 3C, the electrodes 23 are separated from each other by equal distances. The electrodes 22 and 23 are fixed directly onto the tube 21 by a metallization process, such as metal sputtering or deposition. In the case of the electrodes 23, the individual electrodes 23 are defined by etching after the metal is deposited on the outside of the piezoelectric tube 21. Operationally the tubular electrode 21 serves as the common, or ground, electrode. The four electrodes 23 are driven by bias voltages, either positive or negative with respect to ground. The voltage across each of the electrodes 23 and the common electrode 22 causes the portion of the piezoelectric material between the driven electrode 23 and the common electrode to expand or contract in the longitudinal direction. Thus the mirror 13, attached to one end of the piezoelectric tube 21, can be moved toward, away, and tilted with respect to, the mirror 14 by the amount and polarity of the voltages applied to each of the electrodes 23. Figs. 3A and 3B illustrate in an exaggerated manner how the piezoelectric tube 21 is affected by differential voltages across the tube 21. .It should be noted that while three electrodes could be used instead of four, four electrodes provide for a more straightforward control of the alignment of the mirror 13 with respect to the mirror 14.

The tuning function is achieved by varying the distance between the two mirrors 13 and 14. The selected wavelength is tracked by applying a common voltage on all four electrodes 23. After the particular wavelength is found, two pairs of bias voltages, V_x and V_y as shown in Fig. 3C, are applied on opposite electrodes 23 for tilting operations. While each electrode 23 could be operated independently, there is a logical relationship between opposite electrodes. Thus each pair of opposite electrodes 23 receive opposite polarity voltages. Fig. 4 is a block diagram of the controller circuit for the voltages to the electrodes 23 of the piezoelectric transducer assembly 20. The electrodes 23 are driven by a pair of high voltage amplifiers 40_x which generate the V_x voltages to tilt the piezoelectric tube assembly 20 in the x direction and a pair of high voltage amplifiers 40_y which generate the V_y voltages to tilt the assembly 20 in the y direction. (The axes for the x and y directions are in plane of the drawing.) Each amplifier are powered by a high voltage DC power supply 55. Manual or automatic mode of tilt control for each of the x and y directions is selected by a switch 56_x and switch 56_y, respectively. If manual mode is selected for x direction, for example, a manual control unit 50_x responsive to a knob generates control signals, rather than an automatic x-tilt biasing unit 49_x. The control signals from the manual control unit 50_x or the automatic biasing unit 49_x are sent to a first voltage adder circuit 44_x. The circuit 44_x sends its output signa^'ls to a first voltage level transform circuit 45_x, which reverses the polarity of the signal from the circuit 44_x. The signals from the adder circuit 44_x and the level transform circuit 45_x then are sent to separate second voltage adder circuits 42_x. The signals from each of the adder circuits 42_x are then increased by separate second voltage level transform circuits 41_x and passed to the high voltage amplifiers 40_x. The y-tilt control units are similarly arranged as the x-tilt control units. Furthermore, it should be noted that the reversal of voltage polarity by the first voltage level transform circuits 45_x and 45_y allow opposing electrodes 23 work together to tilt the assembly 20. This arrangement avoids the higher voltage (and more expensive) requirements for the amplifiers 4-0_x if the electrodes 23 worked independently.

Manual, sweep and automatic tuning modes for different wavelengths of light through the filter are selected by two switches 57 and 58. The switch 57 selects between the manual mode and sweeping mode; the switch 58 selects between the automatic mode and manual/sweeping -modes. The sweeping mode originates from a sweeping signal generator 47, which has a sawtooth output signal to drive the assembly 20 across the FSR. A manual control unit 48 generates the signal to drive the assembly 20 across the FΞR in response to the position of a knob. An automatic biasing unit 46, which is discussed below, generates an output signal used to track particular signals through the assembly 20. The control signals from the sweeping signal generator 47, t e manual control unit 48, or the automatic biasing unit 46 are sent to a first voltage adder circuit 43 for tuning. The circuit 43 sends its output signals to each of the four, second voltage adder circuits 42_x and 42_y. Through the second voltage level transform circuits 41_x and 41_y, and the high voltage amplifiers 40_x and 40_y, all of the electrodes 23 of the assembly 20 are driven simultaneously to adjust the spacing between the mirrors 13 and 14.

The controller circuit has a dither generator 51, which is selectably connected by a switch 59 to a dither distributor unit 52. The switch 59 is closed for automatic operation. The dither distributor 52 is connected to a channel select unit 53 which, responsive to a timer 54, selects dithering operations for tilting along the x direction, along the y direction, or for tuning (both x and y directions) . Accordingly, the dither distributor unit 52 has an output line connected to the first adder circuit 44_x, the first adder circuit 44_y, and the first voltage adder circuit 43 for tuning. The dither signal to each of these adder circuits 44_x, 44_y and 43 creates a dithering motion in the assembly 20 (and mirror 13) in the x-direction tilt, the y-direction tilt, and the distance between the mirrors 13 and 14, i.e., tuning, respectively.

The dither is used in the automatic mode to control the tilting and tuning adjustments on the piezoelectric tube assembly 20 without intervention. For automatic mode operation, the controller circuit receives a feedback signal from the output signal through the mirrors 13 and 14. The feedback signal is amplified by an amplifier control unit 56, having a level display 57 and a gain control. The amplified signal is passed to a phase detector 58 which also receives a signal from the dither generator 51. In automatic operation, the phase detector 58 combines the dithered output signal and the dither signal itself; the result is filtered by a low-pass filter 60 and sent through a decision unit 60. The decision unit notes the direction of the dither signal by which the dithered output signal is stronger and issues control signals to the x-direction automatic biasing unit 49_x, the y-direction automatic biasing unit 49_y, and the tuning automatic biasing circuit 46. Depending upon which automatic biasing modes are selected, the mirror 13 is tilted and/or tuned toward the stronger signal. If all the automatic modes are engaged, the mirror 13 moves to track the strongest signal, i.e., the peak, near where the automatic tuning mode was started.

Additionally, the mirror 13 automatically tilts in the x and y directions to maintain parallel surfaces with the mirror 14 since signal strength degrades as the mirrors diverge from a parallel state. To ensure the stability of the alignment of the mirrors

13 and 14 , the base 10 is formed from a ceramic base of kovar or invar. These materials have low thermal coefficients of expansion which advantageously reduce the thermal expansion effects in the range of operating temperatures. Likewise, the piezoelectric tube assembly holder 11 is made of machinable ceramic, such as MACOR, a registered trademark of Corning Glass Company of Corning, New York, ceramic is preferred. The holder 11 is mounted to the base 10 by either a machine screw, solder or epoxy. As can be seen in the more detailed view of the piezoelectric tube assembly holder 11 in Fig. 5A, the holder 11 has two cylindrically shaped openings 31 and 32, which start from two opposing surfaces of the holder. The opening 31 interconnect. The opening 32 having the large diameter receives one end of the piezoelectric tube assembly 12, which is held in place by solder or epoxy. The opening 31 having the smaller diameter receives an input fiber/collimator assembly, which is illustrated in Fig. 6.

The mirror holder 15 is formed from machinable ceramic, kovar or invar, with a cylindrically shaped opening 33 connecting two opposing surfaces. The output fiber/collimator assembly are inserted into the opening 33, fixed in place by solder or epoxy.

Mounted to the end of the piezoelectric tube 21 by epoxy is the mirror 13. The mirror 14 is also mounted by solder or epoxy to the surface of the holder 15 so that, the output fiber/collimator assembly in the opening 33 faces the back of the mirror 14. That is, the collimator is near the back of the mirror 14.

Each of the mirrors 13 and 14 are formed from quartz plates, approximately 1—3mm thick. Plates of 1.2mm thickness have been found to work quite effectively. The front and back surfaces of the mirrors 13 and 14 are highly polished. The front surface is covered with reflection coating (98.5% refractivity for a finesse of 100) and the back surface is covered with antireflection coating (99.5% transmission) . Mirror separation depends upon the Free Spectral Range (FSR) . For example, FSR = lOOnm requires that the mirror separation = 12.124 μm for light at λ = 1550nm. The separation is, of course, controlled by the controller circuit described above.

Fig. 6 details the fiber/collimator assembly for each one of the input and output fibers 16 and 17. A quarter-pitch GRIN (GRaded INdex) lens 34 is placed in front of the fibers 16 and 17. The input fiber 16 is shown here as an example. The longitudinal axis of the GRIN lenses 34 is aligned with the longitudinal axis of its corresponding optical fiber 16 and 17. The GRIN lens 34 collimates the light from the input fiber 16,' the GRIN lens in front of the output fiber 17 refocuses the collimated light from the input fiber into the end of the output fiber. Besides GRIN lens, other elements such as aspheric lenses, may be used as collimators for the input and output fibers 16 and 17. Each fiber 16 and 17 are sealed in a cylindrical glass ferrule 35. For improved optical performance, the end of the ferrule 24 and the end of the fibers 16 and 17 are slant-polished so that the tip of the fiber does not end with a surface perpendicular to the longitudinal axis of the fiber. A slight slant is made by polishing the end surface of the ferrule 24 (and the end of the fiber 16 and 17) at a slight angle θ , 8° to 12°, from perpendicularity to the longitudinal axis of the fiber. The ferrule end surface and the fiber end are covered with an anti- reflection coating. Anti-reflection materials, such as MgF₂, Ti0₂ and Si0₂, may be used. The ferrule 35 and the cylindrical GRIN lens 34 are aligned and held in place by a hollow cylindrical holder 36.

With the present invention, bias voltages from 1 to 20 volts, yield tilt angles from 10^"5 to 2X10^"4 radians sufficient to ensure the proper alignment of the mirrors.

While the above is a complete description of the preferred embodiments of the present invention, various alternatives, modifications and equivalents may be used. It should be evident that the present invention is equally applicable by making appropriate modifications to the embodiment described above. For instance, the positions of the common electrode 22 and the separate electrodes 23 could be reversed on the piezoelectric tube 21 so that the common electrode 22 is outside of the electrodes 23. Therefore, the above description should not be taken as limiting the scope of invention which is defined by the metes and bounds of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A tunable optical filter of the Fabry-Perot interferometry type, said filter comprising first and second mirrors having reflecting flat faces facing and substantially parallel to each other; a tubular piezoelectric transducer having a longitudinal axis perpendicular to said mirror flat faces, an inner tubular surface, an outer tubular surface and an end attached to one of said mirrors; a first electrode on one of said inner and outer tubular surfaces of said piezoelectric transducer; and a plurality of second electrodes on the other of said inner and outer tubular surfaces so that said piezoelectric transducer is located between said first electrode and said second electrodes, each second electrode extending parallel to said longitudinal axis and spaced apart from each other in a plane perpendicular to said longitudinal axis; whereby said flat faces of said first and second mirrors may be moved with respect to each other for maintaining said faces parallel to each other and for tuning said filter in response to voltage signals between said first electrode and said second electrodes.

2. The optical filter of claim 1 further comprising an input fiber having an end fixed with respect with to said first mirror for introducing light for reflection between said first and second mirrors parallel to said longitudinal axis within said tubular piezoelectric transducer.

3. The optical filter of claim 2 further comprising an output fiber having an end fixed with respect to said second mirror for receiving light from said reflection between said first and second mirrors, each end of said input and output fibers having a collimator immediately in front of said end.

4. The optical filter of claim 1 wherein said second electrodes are conformally shaped to the other of said inner and outer tubular surfaces.

5. The optical filter of claim 4 wherein said first electrode is on the inner tubular surface of said piezoelectric transducer.

6. The optical filter of claim 4 wherein said first electrode is on the outer tubular surface of said piezoelectric transducer.

7. The optical filter of claim 1 wherein said piezoelectric transducers number four.

8. The optical filter of claim 1 wherein tubular piezoelectric transducer forms a cylinder.

9. A tunable optical filter of the Fabry-Perot interferometry type, said filter comprising a base; first and second mirrors having reflecting flat faces facing and substantially parallel to each other, said first mirror mounted to said base; a transducer assembly mounted to said base, said transducer assembly having a tubular piezoelectric transducer having a longitudinal axis perpendicular to said mirror flat faces, an inner tubular surface, an outer tubular surface and an end mounting said second mirror; a first electrode on one of said inner and outer tubular surfaces of said piezoelectric transducer; and a plurality of second electrodes on the other of said inner and outer tubular surfaces so that said piezoelectric transducer is located between said first electrode and said second electrodes, each second electrode extending parallel to said longitudinal axis and spaced apart from each other in a plane perpendicular to said longitudinal axis; whereby said flat faces of said first and second mirrors may be moved with respect to each other for maintaining said flat surfaces parallel to each other for tuning said filter in response to voltage signals between said first electrode and said second electrodes.

10. The optical filter of claim 9 wherein said base comprises a material having a low coefficient of thermal expansion.

11. The optical filter of claim 10 wherein said material comprises ceramic.

12. The optical filter of claim 9 further comprising a first mirror mount for mounting said first mirror to said base, a transducer assembly mount for mounting said assembly to said base, said first mirror mount and said transducer assembly mount comprising material having a low coefficient of thermal expansion.

13. The optical filter of claim 12 wherein said material comprises ceramic.