WO1988001813A1

WO1988001813A1 - Impedance matched optoelectronic components using lossless elements

Info

Publication number: WO1988001813A1
Application number: PCT/US1987/002006
Authority: WO
Inventors: Jon J. Gulick; Michael De La Chapelle; Hui-Pin Hsu
Original assignee: Hughes Aircraft Company
Priority date: 1986-08-29
Filing date: 1987-08-13
Publication date: 1988-03-10
Also published as: EP0280722A1; AU7968487A; JPH01500713A; ES2005278A6; NO881740L; IL83407A0; KR880702016A; NO881740D0

Abstract

Impedance matching using lossless elements for optoelectronic components. In one embodiment, a fiber optic link has a laser diode (10) optically coupled by an optical fiber (14) to a photodiode (12). The laser diode is matched to the RF source (16) by a reactive matching circuit (M1). The photodiode is matched to the load (18) by a second reactive matching circuit (M2). Both matching circuits are comprised of lossless elements, as opposed to resistive elements, for achieving a match with minimal RF-to-RF insertion loss.

Description

IMPEDANCE MATCHED OPTOELECTRONIC COMPONENTS USING LOSSLESS ELEMENTS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to impedanced matched optoelectronic components. More particularly, the invention relates to an impedance matching scheme using lossless matching elements, such as reactive elements, as compared to the use of resistive

(lossy) matching elements.

2 . Description of Related Art

Optoelectronic components in fiber-optic links for high data rate digital communications are presently available for the telecommunications market. These links generally meet the wide bandwidth needs of that market. For transmission of radio frequency analog signals above about 100 MH_z, however, conventional fiber optic links suffer from several serious drawbacks. Conventional fiber optic links typically use resistive impedance matching (or none at all) to attempt to achieve the bandwidths needed for the transmission of digital signals. For the transmission of analog radio frequency

(RF), microwave or MMW frequency signals this approach is not acceptable. First, the I²R power loss in the resistive matching element results in an unacceptablv large fiber optic insertion loss. Second, the reactive mismatch of the laser diode and photodiode used in the fiber optic link to the external circuit would result in a high input/output VSWR (voltage standing wave ratio) as well as a large link loss at these higher frequencies. The well known characteristics of optical fibers would otherwise make it an attractive alternative to coaxial cable for many commercial and military applications. Among these applications are: the remote placement of command centers from antennas, the distribution of signals for phased arrays, spectrum analysis, delay line signal processing, missile guidance, and analog to digital conversion. In all of these applications fiber optics possess many significant advantages over conventional transmission systems. In order to compete effectively with conventional RF and microwave transmission systems, however, a low loss, impedance matching scheme is needed.

Despite the potential for substantially reducing fiber optic link loss, lossless impedance matching has received little attention. For a review of the literature on impedance matching to a photodiode, refer to K. M. Johnson, "Performance of the Photovoltaic Effect Detector Diode as a Microwave Demodulator of Light", Microwave Journal, pp. 71-75, July 1963. Lossless matching to a laser diode has been discussed in W. E. Stephens, T. R. Joseph, "A 1.3um Microwave Fiber Optic Link Using A Direct Modulated Transmitter", IEEE J. Lightwave Technology, vol. LT-3 pp. 308-315, April 1985. Others have measured laser diode and photodiode microwave S-parameters and derived equivalent circuits for the devices. See K. Y. Lau and A. Yariv, "Ultra High Speed Semiconductor Lasers", IEEE J. Quantum Electronics, vol. QE-21 pp. 121-138 February, 1985; L. Figueroa, C. W. Slayman, and H. Yen, "High-Frequency Characteristics of GaAlAs Injection Lasers," IEEE Trans, Microwave Theory Tech., vol. MTI-30, pp. 1706-1715, Oct. 1982; B. E. Hakki, F. Bosch, S. Lumish, and N. R. Dietrich, "1.3um BH Laser Performance at Microwave Frequencies, "IEEE J. Lightwave Technol., vol. I.T-3, pp. 1193-1200, Dec. 1983; H. Blauvelt, H. Yen, W. B. Bridges, "Fiber Optic Links for Microwave Signal Transmission", to be published in IEEE J. Lightwave Technol; R. S. Tucker, "High-Speed Modulation of Semiconductor Lasers", IEEE J. Lightwave Technol., vol. LT-3, pp. 1180-1192, Dec. 1985; G. Lucovsky and R. B. Emmons, "High Frequency Photodiodes", Applied Optics, vol. 4, pp. 697-702, June 1965; S. Y. Wang, "Ultra-High-Speed Photodiode", Laser Focus/Electro- Optics, pp. 99-106, Dec. 1983. These foregoing referen ces express the high-frequency response limitations of the unmatched device equivalent circuits.

In addition to the literature discussed above, reference may be had to U.S. Patent No. 4,369,525 entitled "Device For Automatic Regulation of the Output Power of a Transmitter Module in an Optical Fiber Transmission System", issued to Breton et al on January 18, 1983, and to U.S. Patent No. 4,257,689, entitled "Pulse Pattern Visual Acuity Device" which issued to Yancey et al on March 24, 1981. The former patent deals with impedance matching using a transistor as an emitter follower. The latter patent details using buffer circuits for impedance matching.

SUMMARY OF THE INVENTION

The present invention is broadly directed to the use of lossless elements for impedance matching in optoelectronic components. In a preferred embodiment there is provided a fiber optic link that uses lossless components, as opposed to resistive (lossy) components, to provide the appropriate transformations at the input to the transmitter and to the output of the receiver. For a given set of optical losses, and a particular laser diode and photodiode (which are used in the preferred embodiment), the use of lossless matching elements results in the minimum obtainable RF-to-RF insertion loss for the link and other advantageous results.

For a more complete understanding of the invention, reference may be had to the following specification, and to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a preferred embodiment of the invention;

FIG. 2(A) is a Smith Chart plot of measured reflection coefficients as a function of frequency for the packaged and intrinsic laser diode;

FIG. 2(B) is an equivalent circuit model for the packaged and intrinsic laser diode plotted in FIG. 2 (A);

FIG. 3 (A) is a Smith Chart plot of measured photodiode reflection coefficients as a function of frequency;

FIG. 3(B) is an equivalent circuit model for the photodiode plotted in FIG. 3(A).

FIG. 4 is a cross-sectional view illustrating a laser diode package;

FIG. 5 (A) is a Smith Chart plot showing a photodiode impedance matching path from the photodiode impedance at. 9.7 GHz to the origin (50 Ohms);

FIG. 5(B) is the lumped element photodiode impedance matching circuit used in FIG. 5 (A);

FIG. 6(A) is the photodiode matching circuit realized on an alumina substrate about .010 inch thick;

FIG. 6(B) is a graph depicting the measured return loss versus frequency for the matched receiver circuit of FIG. 6(A); FIG. 7(A) is a Smith Chart plot showing a laser diode impedance matching path from the laser diode impedance at 9.7 GHz to the origin (50 Ohms);

FIG. 7(B) is a lumped element laser diode impedance matching circuit;

FIG. 8 (A) is a laser diode matching circuit realized on a thicker alumina substrate about .025 inch thick;

FIG. 8 (B) is a graph depicting measured return loss versus frequency for the matched transmitter circuit of FIG. 8(A);

FIG. 9 is a table of fiber optic link losses at

9.7 GHz for various transceiver combinations consisting of lossless/resistive/unmatched transmitters and receivers the results being normalized to the link loss of the lossless matched transceiver; FIG. 10 is a graph depicting link loss versus frequency for matched and unmatched lasers, showing a 5.5 dB improvement as a result of lossless impedance matching; and

FIG. 11 is a graph depicting link loss versus frequency for matched and unmatched photodiodes, showing a 7.5 dB improvement as a result of lossless impedance matching.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

The broad aspects of the invention are illustrated in FIG. 1 in connection with a fiber-optic link using a laser diode 10 and photodiode 12 optically coupled through optical fiber 14. In accordance with the invention laser diode 10 is driven by source 16 through a matching circuit M₁ using lossless elements. For purposes of this invention the term "lossless element" means a device having negligible resistance such as capacitors, inductors, impedance transformers, transmission line (realized as lumped or distributed elements) and the like, as compared with resistive

(lossy) elements. Photodiode 12 supplies the transmitted signal from fiber 14 to a load 18 through matching circuit M₂ which also uses purely lossless elements.

The frequency dependent complex impedance of the laser diode Z_las is transformed to the driving point impedance Z₀₁ of source generator 16 by a purely reactive matching circuit M, in this embodiment. Similarly, the complex impedance of the photodiode Z (pd), which is also frequency dependent, is transformed to the load impedance Z₀₂ via a second reactive matching circuit M₂. These matching circuits M₁ and M₂ can be designed so that the reflection coefficients at the input and output of the fiber optic link ( are 0 at a single

frequency or minimized over a finite range of frequencies. The case of single frequency matching where Z₀₁=Z₀₂=50 Ohms will be used for illustration purposes.

The design of an impedance matched fiber optic link begins with measurements of the laser diode and photodiode reflection coefficients, or S-parameters, as functions of frequency. These are needed for the purpose of microwave characterization and modeling. These measurements can be made on an automatic network analyzer (ANA) from 6 to 12 GHz, for example. These measurements are shown on the Smith Chart plots of FIGS. 2(A) and 3(A). The corresponding FIGS. 2(B) and 3(B) are simple RF equivalent circuit models for the laser diode and photodiode including the device package parasitics. The forward biased laser diode has a low junction resistance in parallel with a diffusion capacitance, both in series with a contact resistance. The reversed biased photodiode consists of a depletion capacitance in series with a contact resistance . The junction resistance is assumed to be infinite and thus excluded from the model. The element values in the equivalent circuit models may be determined by fitting the modeled S-parameters to the measured S-parameters using a computer optimization routine. The element values determined in this fashion are set forth in FIGS. 2(B) and 3(B).

The invention may be implemented using a GaAlAs laser diode such as available from Ortel as component no. LDS10. This diode is depicted in FIG. 4. The package parasitics of this laser diode dominate the measured S-parameters. This makes an accurate determination of the intrinsic device element values difficult, although the laser diode can be repackaged to minimize parasitics and to thereby permit the measurement of the intrinsic S-parameters. The intrinsic model was used in FIG. 2(B). The bond wires were modeled as inductors and the bonding pad as a small section of transmission line. A small resister accounts for interconnect losses. The package parasitics severely limit the impedance matching bandwidth by increasing the frequency sensitivity of the laser diode impedance as shown in FIG. 2 (A). In contrast, the impedance of the intrinsic laser, also shown in FIG. 2(A) varies only slightly with frequency, making it possible to impedance match over a much wider band. Similarly , the photodiode used to implement the invention should have minimal package parasitics. If desired, a 50 micrometer diameter GaAs Schottky photodiode can be employed in chip form to reduce package parasitics. When this is done only a small parasitic inductance caused by the bond wire is included, which does not present a problem.

A negligible change is observed in the laser diode S-parameters for bias currents above threshold

(I_th a≅17mA). Also, the S-parameters are insensitive to optical circuit loading. The change in photodiode scattering parameters with reverse bias voltage results from variations in the depletion capacitance.

There are many ways to match the impedance of the photodiode to the 50 Ohm load impedance. For simplicity, a single reactive element may be used to cancel the imaginary part of the photodiode impedance, followed by a transformer to match the real part of the device impedance. A series inductor L_M resonates the photodiode capacitance C_PD at the center frequency w in accordance with the following equation:

(1) L_M=(1/w²C_PD)-L_B

where L_B is the bond wire inductance.

The photodiode resistance R_PD is matched to 50 Ohms using a quarter wavelength transformer of characteristic impedance:

(2) ^ZT=

The Smith Chart plot of FIG. 5(A) illustrates the impedance matching method and FIG. 5 (B) illustrates the matching circuit for the photodiode. This circuit may be fabricated on a 10 mil thick alumina substrate in order to realize the low impedance (12 Ohms) transformer. FIG. 6 (A) depicts the photodiode matching circuit on a 10 mil alumina substrate. A bias tee, consisting of a series capacitor for DC isolation at the output of the matching network and an inductor in the DC bias path for RF isolation may be included in the receiver circuit.

FIG. 6 (B) also shows the measured return loss versus frequency plot for the fiber optic receiver which exhibits the narrow bandwidth response of this matching circuit. A high return loss indicates that the photodiode is well matched to the load and that maximum power is being transferred. The low impedance quarter wavelength transformer is the bandwidth limiting element in the receiver circuit.

As previously mentioned, commercially available laser diode package parasitics dominate the measured reflection coefficients, so that essentially one ultimately matches to the package parasitics rather than to the intrinsic device itself. However, one may repackage the laser to minimize unwanted parasitics. A simple two-element narrow band impedance match between the source and the packaged laser diode may be employed. As seen from the Smith Chart plot of FIG. 7(A), a small series inductance followed by a shunt capacitance has the effect of rotating the measured reflection coefficient to the unity conductance circle and then along this circle to the origin.

These matching elements may be realized as distributed elements on a 25 mil thick alumina substrate, as shown in FIG. 8(A). The frequency dependence of these distributed elements is negligible when compared to the narrow band response of the laser package. The return loss characteristic of this impedance matched fiber optic transmitter is also shown in FIG. 8(B). The response is very narrow band and extremely sensitive to the matching circuit elements. A tuning chip may be required to center the response at the designed frequency.

Commercially available fiber optic transievers for low data rate transmission are generally not impedance matched at the input or output. However, for gigabit digital and GHz analog signal transmission, impedance matching is necessary in order to reduce link loss and to eliminate high VSWRs at the input and output.

The mismatch loss of the laser diode and photodiode is given by the ratio of available power delivered to a matched to the power delivered to an unmatched load, and may be expressed as:

(3)

where p_D is the reflection coefficient of the diode.

(4)

In equation (4 ) Z_D is the device impedance and R₀ is the source and load impedance. The link loss improvement at 9.7 GHz as a result of lossless matching photodiode is found by substituting the measured value of Z_D into equations (3) and (4). For Z_D= (2.7-j17.6) Ohms, the result is 7.6 dB. Similarly, the expected improvement in link loss due to lossless matching of the laser at 9.7 GHz is calculated using the measured value of Z_D=(6.21-j35.6) Ohms to get 5.5 dB. Therefore, the total reduction in link loss of the lossless impedance matched link over the unmatched link is 13.1 dB.

The Ortel LDS10 laser described above is also available in a package with an integral SMA connector. This laser is designed to be driven from a 50 Ohms source through the use of an internal series resistor. The internal matching resistor measures 46 Ohms. This technique of impedance matching is adequate at low frequencies resulting in a low input VSWR to the laser. However, it is inadequate at high frequencies when the reactance of the laser diode becomes large. In addition, most of the input power to the fiber optic transmitter is dissipated in the 46 Ohms resistor, thereby resulting higher link loss. The relative improvement in link loss of the lossless matched fiber optic transmitter over the transmitter with a lossy series matching resistor R_S is given by:

(5)

where the laser impedance is Z_L=R_L+jX_L. Substituting the measured value of Z_L= ( 6.21-j35.6) Ohms at 9.7 GHz into equation (5) gives an anticipated link loss improvement of 9.7 dB by lossless matching over resistance matching. These results are summarized in FIG. 9 which shows the overall link losses for various combinations of lossless and resistively matched fiber optic transmitters and receivers. The values are normalized to the link loss of the lossless matched fiber optic transceiver.

The measured relative improvement in fiber optic link loss as a result of lossless impedance matching the laser diode over that of an unmatched laser diode is shown in FIG. 10. The measurement was first made taking data for a link consisting of an unmatched Ortel LDS10 laser diode transmitter along with an unmatched photodiode receiver. The unmatched transmitter was then replaced with a matched transmitter and the link loss versus frequency was again measured. The measured improvement in link loss due to lossless matching of the laser at the center frequency was 5.5 dB, which agrees with the result predicted by equation (3). The laser was operated in the roll-off region (above the relaxation resonance frequency) demonstrating that the matched circuit can be used to compensate for the roll-off in the high frequency response of the laser. Similarly, the lossless matched receiver demonstrates a 7.5 dB improvement in link loss at the center frequency over the unmatched receiver (see FIG. 11). This measured result also closely agrees with the result derived from equation (3).

From the foregoing it will be understood that the invention provides optoelectronic components which employ purely lossless elements, as opposed to resistive elements, to provide the appropriate impedance matching transformations, for example, at the input to the laser diode transmitter and at the output of the photodiode receiver. While the invention has been described in accordance with its presently preferred embodiments, it will be understood that the invention is capable of certain modification and change without departing from the spirit of the invention as set forth in the appended Claims. For example, while this invention has particular utility in a fiber optic link for microwave transmission, the broad teachings can be used to match the impedance of almost any optoelectronic device operating at frequencies of at least 100 MHz . The lossless elements can be fabricated on a variety of media, preferably microwave medium such as the dielectric substrates noted above, waveguide, coax, microstrip, stripline, coplanar line and the like. Other modifications will become apparent to the skilled practitioner upon a study of the drawings , specifications , and following claims.

Claims

CLAIMS What is claimed is:

1. Apparatus comprising an optoelectronic component and an impedance matching circuit using solely lossless elements.

2. The apparatus of Claim 1 wherein the lossless elements are selected from the group of capacitors, inductors and impedance transformers.

3. The apparatus of Claim 2 wherein the optoelectronic component is a laser diode.

4. The apparatus of Claim 2 wherein the optoelectronic device is a photodiode.

5. The apparatus of Claims 3 or 4 wherein the optoelectronic component is operated at a frequency of at least 100 MH_z .

6. The apparatus of Claim 5 wherein the lossless elements are disposed on a microwave transmission medium selected from the group of dielectric substrates, waveguide, coax, microstrip, stripline, and coplanar line.

7. The apparatus of Claim 6 wherein the medium is an alumina substrate.

8. A method of making apparatus operating at frequencies of at least 100 MH_z, said method comprising: providing an optoelectric component; measuring characteristics of the component at given frequencies; selecting values of lossless elements to match the complex impedance of the component at a selected frequency; and fabricating the elements on a microwave medium.

9. The method of Claim 8 wherein the component is a packaged laser diode transmitter and the measured characteristics include reflection coefficients thereof.

10. The method of Claim 8 wherein the component is a photodiode receiver and wherein the measured characteristics include reflection coefficients thereof.

11. The method of Claims 9 or 10 wherein the lossless elements include a distributed capacitor and inductor on a dielectric substrate.

12. A fiber optic link comprising: a source of energy providing signals having a frequency of at least 100 MH ; a laser diode for converting said signals from the source into optical energy, said laser diode having a complex impedance comprising a real part and an imaginary part; a first matching circuit for coupling said source to said laser, said first matching circuit transforming the source impedance to the laser diode impedance using lossless elements; a photodiode for receiving optical energy, said photodiode having a complex impedance comprising a real part and an imaginary part; means for optically coupling said laser diode and said photodiode; a second matching circuit for coupling said photodiode to a load, said second matching circuit transforming the photodiode impedance to the load impedance using lossless elements; whereby a minimum insertion loss between said source and said load is achieved.

13. The fiber optic link of Claim 12 wherein said laser diode comprises a semiconductor laser diode.

14. The fiber optic link of Claim 13 wherein said laser comprises a gallium arsenide laser diode.

15. The fiber optic link of Claim 12 wherein said first reactive matching circuit comprises a series inductance means.

16. The fiber optic link of Claim 12 wherein said first reactive matching circuit comprises a series inductance and shunt capacitance.

17. The fiber optic link of Claim 12 wherein said first reactive matching circuit is disposed on a dielectric substrate.

18. The fiber optic link of Claim 12 wherein said second reactive matching circuit comprises a series inductance.

19. The fiber optic link of Claim 12 wherein said second reactive matching circuit comprises a series inductance and a transformer.

20. The fiber optic link of Claim 19 wherein said transformer is a quarter wavelength transformer.

21. The fiber optic link of Claim 12 wherein said second reactive matching circuit is disposed on a substrate of alumina.