Peripheral coupled traveling wave electro-absorption modulator

A method for optical modulation comprising the steps of guiding an optical wave in an optical waveguide, the optical wave having an evanescent tail; and applying a modulation voltage to the evanescent tail.

TECHNICAL FIELD

The invention is in the optoelectronic field. The invention is applicable to optical modulation systems.

BACKGROUND ART

Optical modulators are used in a variety of applications. Controlled modulation of light is useful in analog systems to produce an output proportional to the input signal. Digital optical systems, such as fiber optic communication systems, use optical modulators to impose digital signals on light. Digital optical modulators as signaling devices may also form the basis for optical memories and general computer devices.

One form of optical modulation is electro-absorption (hereinafter, “EA”) modulation. In conventional EA modulation, EA material is an integral part of the optical waveguide. Consequently, the design of the microwave waveguide is constrained by the optical waveguide design. It is necessary to trade off optical and microwave waveguide design considerations.

As a result, after considering various trade-offs, existing optimized EA modulators are typically 200 μm long or shorter, and the EA layer is a few thousand angstroms thick over the width of the waveguide. At such short interaction lengths, they do not take full advantage of traveling wave interactions. The size of the optical mode is approximately 1 to 2 μm, requiring expansive and precise coupling to single mode optical fibers. The high density of the optical field in the EA layer of an EA modulator of such a small mode also limits the saturation optical power of the modulator typically to a few milliwatts.

DISCLOSURE OF INVENTION

An embodiment of the present invention is directed to a method for optical modulation comprising the steps of guiding an optical wave in an optical waveguide, the optical wave having an evanescent tail; and applying a modulation voltage to the evanescent tail.

BEST MODE OF CARRYING OUT THE INVENTION

Broadly stated, embodiments of the invention use peripheral coupling of a microwave wave and an optical wave. With the invention, strong EA modulation may be achieved. Embodiments of the invention may achieve number of benefits, including separation of design optimization for optical waveguides and microwave waveguides working together to modulate an optical wave; provision of a millimeters-long synchronized length for interaction between a microwave wave and an optical wave obtaining a very low modulation voltage; microwave transmission line design with low attenuation and impedance matching to the source; relatively easy optical coupling to fibers; and large optical saturation power compared to other EA modulators.

Turning now toFIG. 1, showing a schematic cross-section of an embodiment of the invention, an apparatus for optical modulation includes an optical waveguide10and a microwave waveguide12. Microwave waveguide12has an EA material14sized and placed such that, for an optical wave of interest guided in optical waveguide10, EA material14is located in an evanescent region16, a region occupied by the optical wave's evanescent tail when the optical wave of interest is being guided in optical waveguide10.

Optical waveguide10includes substrate18, an N-doped upper semiconducting cladding layer20, a semiconducting core layer22disposed between substrate18and upper semiconducting cladding layer20, and a lower semiconducting cladding layer24disposed between substrate18and semiconducting core layer22. A heavily doped N-contact layer26is disposed on upper semiconducting cladding layer20, and N-contact layer26and the upper part of upper semiconductor cladding layer20and are etched to form a ridge for optical waveguide10.

Semiconducting core layer22has a higher index of refraction than that of lower semiconducting cladding layer24and of upper semiconducting cladding layer20. This structure provides vertical confinement of an optical wave in optical waveguide10. The ridge structure of N-contact layer26and the upper part of upper semiconducting cladding layer20provides lateral confinement of the optical wave in optical waveguide10.

Optical waveguide10and microwave waveguide12share N-contact layer26within the ridge structure. Microwave waveguide12further includes two N-contacts28, which are disposed on an upper surface of N-contact layer26at the outer edges of that upper surface.

Microwave waveguide12further includes an EA material14disposed on N-contact layer26between the two N-contacts28, a P-contact layer30disposed on EA material14, and a P-contact32disposed on P-contact layer30. EA material14may be formed from a Group III–V compound material. One embodiment of the invention uses InGaAsP for EA material14. Another embodiment of the invention uses GaInAlAs for EA material14.

When guided in optical waveguide10, an optical wave of interest is primarily in semiconductor core layer22, but it also extends into lower semiconductor cladding layer24, upper semiconductor cladding layer20, N-contact layer26, EA material14, and beyond. Most of the optical intensity of an optical wave of interest when guide in optical waveguide10is located in a main mode that occupies main mode region34, and the amplitude of the optical wave decays as it extends further away from semiconductor core layer22. The part of the decaying optical wave in lower semiconducting cladding layer24, the upper semiconducting cladding layer20, N-contact layer26, and EA material14is called the evanescent field, evanescent wave, or evanescent tail. The region in which the evanescent tail is present when an optical wave is being guided in optical waveguide10is shown as evanescent region16. As the optical properties (i.e., the absorption coefficient and the refractive index) of EA material14are changed by the electric field produced by the modulation voltage applied to the microwave waveguide12, the optical properties of EA material14in turn affect the propagation of the optical wave in optical waveguide10through the evanescent tail in evanescent region16, enabling the modulation of the optical wave by the microwave voltage. The coupling of EA material14in the microwave waveguide12to the modulation of the optical wave in the optical waveguide10via the evanescent field in evanescent region16constitutes the peripheral coupling of the microwave waveguide12and the optical waveguide10.

FIG. 2shows a schematic cross-sectional view of another embodiment of the invention. The structure of the substrate18, lower semiconducting cladding layer24, and semiconducting core layer22is the same as inFIG. 1. In this embodiment of the invention, upper semiconducting cladding layer20and N-contact layer26are etched differently to form a different contact structure and a different ridge structure for lateral confinement of the optical waveguide10mode. Two N-contacts28for the microwave waveguide12are disposed on N-contact layer26, one on either side of main mode region34and evanescent region16of optical waveguide10. N-contact layer26and upper semiconducting cladding layer20are etched away between each N-contact28and main mode region34and evanescent region16of optical waveguide10to form a ridge for lateral confinement of an optical wave in optical waveguide10. Optimizations of embodiments of the invention will place the N-contacts28relatively far away from the ridge structure of upper semiconducting cladding layer20and N-contact layer26. In one embodiment of the invention, the N-contacts28are disposed at each edge of the etched-away areas opposite the ridge formed by the etched-away areas.

On the ridge, N-contact layer26is shared by optical waveguide10and microwave waveguide12in this embodiment of the invention. Microwave waveguide12includes N-contacts28disposed on N-contact layer26as discussed above and a structure on the ridge of optical waveguide10that includes EA material14disposed on N-contact layer26, P-contact layer32disposed on either side of a top surface of EA material14, insulators35on either side of EA material14and P-contact layer32, and a truncated “V”-shaped P-contact36with the truncated tip of the “V” in contact with EA material14, disposed between either side of P-contact layer32and between insulators35. Insulators35may be made of polyimide, for example.

Use of truncated “V”-shaped P-contact36surrounded by insulators35reduces the capacitance of microwave waveguide12. A relatively thick (referring to the vertical dimension inFIG. 2) truncated “V”-shaped P-contact36reduces microwave loss in microwave waveguide12. The tip of truncated “V”-shaped P-contact36increases the electric field in EA material14. An approximate 5.0×106V/m strength is necessary for modulation. This may be achieved by all inventive embodiments. TheFIG. 2embodiment achieves high field strengths at especially low drive voltages. For example, at a drive voltage of 1 V, an electric field of 1.0×107V/m may be achieved in parts of EA material14. In an embodiment of the invention, the tip of truncated “V”-shaped P-contact36need be only 0.5 μm wide, but the width and position of the tip do not need to be maintained with a high degree of accuracy.

In an embodiment of the invention, EA material14is a multiple quantum well material. EA material14typically consists of several quantum wells. For instance, for 1550 nm wavelength modulation, EA material14may be a five-quantum-well stack each of which is made of an InGaAsP well (optimally 100 Å thick with a bandgap energy of 0.8 eV) and an InGaAsP barrier (optimally 70 Å thick with a bandgap energy of 1.08 eV). In another embodiment of the invention, EA material14is made of Franz-Keldysh effect materials, e.g., InGaAsP that is 1000 Å thick with a bandgap energy of 0.85 eV, optimized for 1550 wavelength modulation.

One of the benefits of embodiments of the invention is that those embodiments permit separation of design optimization for optical waveguide10and microwave waveguide12working together to modulate an optical wave. A discussion of certain design considerations permits description of preferred embodiments of the invention, using the exemplary embodiments illustrated inFIGS. 1 and 2among several embodiments.

Let z be the direction of propagation of optical waveguide10and microwave waveguide12. Iois the incident optical power in optical waveguide10at the input (z=0) and I(z=L) is the transmitted optical power in optical waveguide10at the output end (z=L). The microwave wave field in EA material14is given by the microwave voltage at z, VRF(z), divided by di,eff, the effective thickness of EA material14. For microwave waveguide12at low frequencies, di,effis approximately the physical thickness of EA material14. At high frequencies, di,effmay be larger than the physical thickness of EA material14and may be determined from microwave field analysis. The transmission function of any traveling-wave EA modulator (hereinafter, “TWEAM”) in response to a continuous-wave microwave voltage VRcos ωt at z=0 is:
I(z=L)/Io(z=0)=T=ηins·eΓαbiasL·e−ΓΔαeff(ΔF)L(1)
whereΓ=optical confinement factor of EA material14;ηnis=insertion efficiency=CinCout(1−R)2e−αoL;

A modulation voltage ΔF will create a Δαeffthat will change transmission T. The optimization of the Δα (as that measured from the biasing voltage) by the ΔF is primarily a materials issue. In addition, modulation of T is affected by L, Γ, ηins, αbias, δ, αrf, and di,eff.

When microwave waveguide12is perfectly impedance matched at its input and the output ends and when there is no microwave propagation loss, VRFis just a constant (half of the microwave source voltage). When there are mismatches at the input and output end or attenuation, VRFis a function of z that consists of attenuated forward and backward propagating waves. Described herein is the effect of microwave attenuation as it reduces the magnitude of VRFas z increases from 0, without describing VRF(z) mathematically. The insertion efficiency ηinsconsists of the coupling efficiency to the laser or the fiber at the input and the output (CinCout), the Fresnel reflections at the input and the output ((1−R)2) and the optical wave residual propagation loss (e−αoL, excluding the absorption due to the EA effect). e−ΓαbiasLrepresents the reduction of the transmission T due to the EA effect of the bias voltage. At zero bias voltage, e−ΓαbiasL=1.

Equation (1) describes a modulation voltage that has a time variation of cos ωt. In that case, matching of nmicrowaveand noptical(i.e., matching of the microwave and optical phase velocities or δ=0) will yield the largest Δαefffor a given αrfand VRF/di,eff. For pulse modulation, Eqn. (1) will be modified. In that case, the matching of the optical and microwave group velocities will achieve the most effective modulation. Clearly, the most effective Δαefffor a given drive voltage is obtained when there is the smallest di,eff, least microwave attenuation, best matching of phase and/or group microwave and optical velocities and best impedance matching of microwave waveguide12to the microwave driver. In addition, the smaller the Γ, the smaller the density of the optical radiation in EA material14, and the larger the saturation limit of the total optical power modulated by embodiments of the invention. The larger the optical mode, the smaller the propagation loss of optical waveguide10caused by scattering, and the more conveniently embodiments of the invention may be coupled efficiently to single mode optical fibers.

In digital applications, the bias voltage for the on-state is normally zero. Thus, Ion=IoT=Ioηinsat the on-state. In an embodiment of the invention, CinCoutis maximized, R is minimized, αrdis minimized, and αois minimized. The maximum L that can be used will depend on the insertion loss allowed, CinCout, R, and the residual propagation loss αrfand αo. At the off-state, the output power is Ioffand
Ioff/Ion=extinction ratio=e−ΓΔαeff(ΔF)L(2)

The most effective modulator would have the smallest VRFthat must be used to achieve a given required extinction ratio, requiring the most sensitive Δα(ΔF) and the largest ΓL in optical waveguide10, plus the smallest di,effin the microwave waveguide12. To obtain large Δαefffor a given di,effand a given Δα(ΔF), the best group velocity matching, the least microwave attenuation, and the best matching to the driver circuit are required in microwave waveguide12. Much better overall performance can be obtained by using small Γ and large L (L will be limited by αrfand αo) in embodiments of the invention. The Γ is kept as large as possible as long as the optical power is sufficient for the application, and the microwave/optical coupling configuration is sufficiently weak to achieve the microwave objectives (very small di,eff, low attenuation, plus velocity and impedance matching) without affecting seriously the optical design that gives large ηins, relatively easy coupling, and large L. Embodiments of the invention may place microwave waveguide12away from the center of a ridge structure of N-contact layer26and the upper part of upper semiconducting cladding layer20to reduce αo. A result of embodiments of the invention is a large ΓL as well as a large Δαeff, using small drive voltage.

When the Franz-Keldysh effect is used for EA, e−ΓΔαeffLwill be less sensitive to optical wavelength change. A Franz-Keldysh peripheral coupling TWEAM may be designed to achieve a minimum extinction ratio for a group of wavelengths in Wavelength Division Multiplexing (“WDM”) applications. Since embodiments of the invention allow microwave waveguide12be placed on one side of optical waveguide10, other optical structures such as a periodic structure may also be added to optical waveguide10from the top to achieve desired chirping effects. Novel structures for EA material24such as inner barrier step quantum well (“IQW”) material may also be used to control chirping effects.

In analog applications, the modulation voltage is a small signal to the bias voltage. The criteria used to measure the link performance (with matched impedance at the input and the output) is the RF gain under a given bias condition,
GRF=(Io·ηins·∂T/∂V·ηdet)2·Rin·Rin·Rout(3)
where ηdetis the detector efficiency, V is the input RF modulation voltage, and Rinand Routare the source and load resistance at the detector. Under a given bias condition,

Here the modulation field in EA material24is Fm, Fm=Vm/di,eff. Vmis produced by the RF drive voltage V. Dependent on the αoand αbias, there is a value of optimum L that maximizes ∂T/∂Vm. In addition, αbiasand ∂Δαeff/∂Fmalso vary as the bias voltage is varied. The best RF gain is obtained with the highest ηins, the largest Io, and the largest ∂T/∂V. ∂T/∂V is maximized by the optimum ΓL and the smallest di,eff. Embodiments of the invention permit use of small Γ and large L to obtain the optimum ΓL. Io, limited by saturation, can be increased by reducing Γ. High ηinswith relatively easy coupling is obtained by using a large optical mode. An optimal design of microwave waveguide12should yield the smallest di,effand the largest ∂Δαeff/∂Fm. Since ∂T/∂Vmcontains e−αol, the design of optical waveguide6should have αoL<<αbiasΓL. When αoL<<αbiasΓL, the maximum ∂T/∂Vmoccurs approximately at e−biasΓL=0.5. At this optimum ΓL the maximum ∂T/∂Vmdepends only on di,eff, αbiasand ∂Δαeff/∂Fm. Besides RF gain, the other important consideration in analog applications is the reduction of non-linear distortion. A number of techniques for reduction of non-linear distortion may be realized in embodiments of the invention.

Since embodiments of the invention allow high ηinsand large ∂T/∂V, GRF>1 may be obtained at large Io. In that case, wide bandwidth RF amplification may be achieved that cannot be obtained electronically. In principle, such a RF amplifier may be integrated on the same chip. As with embodiments of the invention for digital applications, embodiments of the invention using the Franz-Keldysh effect may be used for various adjacent wavelengths with the RF gain controlled by adjustment of bias voltage.

While various embodiments of the present invention have been shown and described, it should be understood that modifications, substitutions, and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions, and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.