WO2004042881A2

WO2004042881A2 - Optical frequency comb generator

Info

Publication number: WO2004042881A2
Application number: PCT/GB2003/004848
Authority: WO
Inventors: Alwyn Seeds
Original assignee: University College London
Priority date: 2002-11-08
Filing date: 2003-11-10
Publication date: 2004-05-21
Also published as: GB0226154D0; AU2003301784A1; WO2004042881A3; AU2003301784A8

Abstract

An optical frequency comb generator comprises a quantum confined phase modulation (QCPM) component driven in use by an electrical modulating signal; and a laser gain component coupled to said QCPM component and the electrical modulating signal selected to generate an optical comb spectrum.

Description

OPTICAL FREQUENCY COMB GENERATOR

1. FIELD OF THE INVENTION

This invention relates to the fields of optical frequency comb (OFC) generation, Frequency Modulation (FM) lasers, electro-optic phase modulation, and more particularly, to an OFC generator using an FM laser with quantum confined phase modulation component.

2. REVIEW OFTHEARTKNOWNTOTHEAPPLICANTS

In recent years, there has been increasing interest in exploring optical frequency comb (OFC) generation for application in optical communications and precision measurements. Optical comb generators are key elements in many applications such as all-optical network channel generation and microwave signal synthesis. The large number of precisely spaced sidebands makes an OFC attractive for use in dense wavelength division multiplex (DWDM) optical communication networks and can offer reduced system cost. The optical comb can be used for identifying channel frequencies and locking transmitter and receiver light sources with optical frequencies exactly determined by the comb signal. Requirements on comb generators for DWDM networks involve absolute frequency accuracy, exact spacing, narrow linewidth, wavelength tuneability, good stability of frequency comb and uniform comb line power with optical frequency. Atomic or molecular resonance is commonly used for providing absolute frequency reference at a few particular frequencies, but is not available at each required channel frequency. Optical sources locked to a Fabry-Perot (FP) interferometer or fibre ring resonator provide a regularly spaced optical frequency comb, but the frequency range, accuracy and stability are limited by the unstable optical length of the resonators. Precisely spaced frequency lines can be generated by phase modulation of an optical source. However, due to difficulties in realising phase modulation at high frequencies, both the number of frequency lines and their spacing are severely limited. Approaches based on the use of separate sources with periodic wavelength measurement and correction, are unwieldy where a large number of allocable channels are required to obtain routeing flexibility. In earlier work, the comb spacing was limited by the modulation methods used and was far too small for many applications. Tuneability of operating wavelength and comb spacing is also restricted.

Several types of OFC generation techniques have been developed. These involve absolute frequency referencing described in reference [1], FP interferometric OFC generators described in ref.[2], fibre ring resonator frequency comb generators detailed in reference [3], and large index phase modulation comb generators described in ref. [4].

A frequency modulation (FM) laser is another potential OFC source. It is a laser having an FP or ring cavity containing an intra-cavity phase modulator so that the laser can exhibit frequency-swept mode of operation representing a different type of mode coupling from the short-pulse operation of the laser. A large number of comb lines spaced by the modulator drive frequency are generated when the modulation frequency is detuned by a small amount from the exact axial or ring cavity mode spacing. Theoretical and experimental analyses of FM laser operation are described in reference [5]. An external cavity configuration has been extensively used for FM lasers. This is analysed in references [6-7] where the FM laser comprises a FP laser diode with an embedded phase modulator in an external cavity, and the authors present analysis on how modulation frequency tuning leads to pulsed or FM operation of the laser. In the FM laser the phase modulator is an important component in realizing FM laser operation. In previous work there have been several types of phase modulator components developed. In reference [8], the authors demonstrated use of a solid-state electro-optic (EO) crystal, LiNbO₃, as a phase modulator for the FM laser. Solid-state EO modulators have been extensively used in solid-state FM lasers, gas FM lasers and fibre FM lasers. The problem with this type of modulator is that it is generally bulky and so difficult to integrate in a short enough laser cavity to give the comb line spacings of 5GHz-50GHz typically required for DWDM optical fibre communication networks. High driving voltages are also generally required which become difficult to achieve at high modulation frequencies. Another type of phase modulation component is the semiconductor phase modulator operating with forward bias current injection, which leads to carrier-induced phase modulation. Spectroscopy and refractive index change has been investigated in reference [9]. There are disadvantages for the current modulation phase modulator for FM laser operation. One is the thermal effect that leads to a large change in modulation response with modulation frequency. The other is relatively narrow modulation bandwidth of less than 1GHz, which leads to difficulty in developing comb spacings large enough (>5GHz) for DWDM optical communication networks.

3. SUMMARY OF THE INVENTION

In contrast, quantum confined phase modulation (QCPM), for example phase modulation due to the quantum confined Stark effect (QCSE) has exhibited better characteristics including low optical attenuation, large refractive index change, low driving voltage and operating speed fast enough to achieve phase modulation at frequencies extending into the millimetre- wave region.. Investigation of the above fields has shown that a novel optical comb generator suitable for DWDM optical communication can be realized using an FM laser incorporating quantum confined phase modulation techniques.

The present invention comprises an optical frequency comb generator using FM laser operation that comprises a laser gain component and a semiconductor QCPM phase modulation component within the laser cavity.

The defined laser gain component is one of lasers or laser arrays that can be coupled with the QCPM modulator to produce FM laser operation.

The defined QCPM phase modulator is a semiconductor phase modulation component comprising an active region of quantum wells, or quantum wires or quantum dots, operating with quantum confined phase modulation, for example with quantum confined stark effect (QCSE) phase modulation, and comprising either a waveguide structure or a vertical cavity structure. A constant reverse bias voltage and a modulation signal of frequency equal to the desired comb line frequency spacing is applied to the said QCPM phase modulator.

An alternative for the defined OFC generator is monolithic integration of the system, in which the defined QCPM modulator and the laser gain component are integrated on a common substrate.

An alternative for the defined OFC generator is an external cavity OFC generation system comprising a FP cavity defined by two optical mirrors. The said optical mirrors of the FP cavity are reflection coated for the range of optical wavelengths occupied by the comb spectrum. All other defined components within the cavity have anti-reflection coatings on the surfaces or are otherwise configured to suppress reflections.

Another alternative for the defined OFC generator is a ring cavity OFC generation system defined by free space propagation and mirrors or by waveguiding, for example in optical fibre or planar glass, semiconductor or polymer. All other defined components within the cavity have anti-reflection coatings on the surfaces or are otherwise configured to suppress reflections.

The defined QCPM phase modulator may use an air-bridged contact or other low parasitic capacitance contacting techniques so that an electric field can be applied to the modulator with fast response.

It is an object of this invention to provide an OFC generator based on FM laser operation with QCPM phase modulation, comprising a laser gain component and a QCPM phase modulation component driven with an electrical frequency equal to the required comb frequency, producing an optical frequency comb with spacing suitable for DWDM applications.

Another object of this invention is to provide an alternative of the defined OFC generator, comprising a monolithically integrated OFC generator where the defined QCPM modulator and the laser gain component are combined on a single substrate.

Another object of this invention is to provide an alternative of the defined OFC generator, comprising an external FP cavity defined by two mirrors with reflecting coatings.

Another object of this invention is to provide an alternative of the defined OFC generator, comprising a ring cavity. These and other features, advantages and objects of the invention will be apparent from the following detailed description, which description makes reference to the following drawings.

4. BRIEF DESCRIPTION OF DRAWING

Fig.1 is a schematic diagram of the present invention about a general optical frequency comb generator using an FM laser incorporating the QCPM modulator.

Fig.2 shows a monolithic integration of this invention.

Fig.3 shows a FP external cavity configuration of this invention.

Fig.4 shows a ring cavity configuration of this invention.

Fig.5 shows one preferred embodiment of this invention, where a coupling optical component is applied for enhancing light coupling efficiency between the laser gain section and the phase modulator. The phase modulator is a quantum confined Stark effect (QCSE) phase modulator. Constant reverse bias and a modulation signal are applied to the QCSE phase modulator. An optical frequency comb spectrum is generated with this system.

Fig.6 is one embodiment of the QCPM modulator for use in the invention, which operates with multi-quantum well (MQW) QCSE phase modulation having light incident perpendicular to the plane of the quantum wells and air-bridged p contact on the device. Fast modulation response is demonstrated with 3dB cut-off frequency of over 6GHz and the device is light polarization independent.

Fig.7 shows -3dB FM bandwidth over a range >6GHz with the embodied MQW QCSE phase modulator defined by Fig.6.

Fig.8 is a comb spectrum generated by the embodied OFC generator defined by Fig. 5, which incorporates the embodied QCSE phase modulator defined by Fig.6.

Fig.9 shows frequency modulation comb bandwidth generated by the embodied OFC generator defined by Fig.5, incorporating the embodied QCSE phase modulator defined by Fig. 6., as a function of modulation signal power applied to the QCSE phase modulator.

6. DESCRIPTION OF THE PREFERRED EMBODIMENTS

A general configuration of this invention can be understood by reference to Fig.l, wherein a laser gain section 1, which can be one of lasers or laser arrays suitable for coupling with the phase modulator to produce FM laser operation, and a quantum confined phase modulation component driven by an electrical modulating signal of frequency equal to the required comb line spacing 2 coupled with the laser gain section within the laser cavity to operate in FM laser mode and generate an optical comb output 7. The facet 3 of the phase modulator and the facet 4 of the laser gain section are anti-reflection coated. The facet 5 of the laser gain section and the facet 6 of the phase modulator are reflection coated. In Fig.2, as an alternative to the OFC generator defined by Fig.l, a monolithically integrated configuration of the system is shown. In this system, the laser gain section 10 is integrated with the QCPM phase modulation component 11 to form a compact OFC generator. The comb light 14 can be coupled out via either the facet 13 of the laser gain section or the facet 12 of the QCPM modulator, for example, in Fig.2, the comb light is coupled out from the front facet 13 of the laser gain section. The facets 12, 13 of the phase modulator and the laser gain section are reflection coated, and the integrated interface 15 is a high transmittance interface.

In Fig.3, as an alternative to the OFC generator defined by Fig.l, a FP external cavity system is shown. In this system, an FP external cavity is defined by two mirrors 24, 25. The laser gain section 20 and the QCPM phase modulator 21 are embedded in the FP external cavity and aligned along the cavity axis. Comb light 26 is coupled out with the mirror 25. The end facets 22, 23 of the QCPM modulator and the laser gain section are anti-reflection coated, while the integrated interface 27 is a high transmittance interface. The mirrors 24, 25 are optical mirrors with reflective coatings. The laser gain section and the QCPM modulator can also take another form of separate alignment in the cavity.

In Fig.4, as another alternative to the OFC generator defined by Fig.l, a ring cavity system is shown. In this system, a ring cavity is defined by three mirrors 34, 35 and 36. Variation in the ring cavity shape with more mirrors is also possible. The laser gain section 30 and the QCPM phase modulator 31 are embedded in the ring cavity and aligned along the cavity axis. Comb light 37 can be coupled out with one of the mirrors 34, 35, 36, for example in the configuration of Fig.4, with mirror 35. The facets 32, 33 of the QCPM modulator and the laser gain section are anti-reflection coated, while the integrated interface 38 is a high transmittance interface. The mirrors 34, 35, 36 are optical mirrors with reflective coatings. The laser gain section and the QCPM modulator can also take another form of separate alignment in the cavity and the cavity can be formed in waveguide components.

Figure 5 shows a preferred embodiment of the OFC generator defined in Fig.l. The laser gain section is an InGaAsP/InP multi-quantum well (MQW) FP laser 100, of which the front facet 109 is reflection coated for the range of wavelengths occupied by the comb spectrum as a cavity mirror and the rear facet 110 of the gain laser is anti-reflection coated over the range of wavelengths occupied by the comb spectrum. The anti-reflection coating on the facet 110 is deposited using Si₃N₄ material and results in a reflectivity less than 0.1%. The QCPM phase modulation component is a fast MQW QCSE phase modulator with light incident normal to the plane of the quantum wells 101, as shown in Fig.5, incorporating a monolithically integrated high reflectivity DBR mirror 112 below the MQW region. The front facet 111 of the QCSE modulator 101 is also anti-reflection coated over the range of wavelengths occupied by the comb spectrum. The DBR mirror 112 of the modulator 101 and the front facet 109 of the FP gain laser 100 form the cavity of the FM laser. An optical lens with anti-reflection coating over the comb spectrum on both sides of it is included between the FP laser gain section 100 and the QCSE modulator 101. A constant reverse bias 105 is applied to the QCSE modulator 1 1 to produce phase tuning. To make the system generate comb lines, a microwave modulation signal from a source 104 is applied to the QCSE modulator 101 via a triple stub tuner 106 and a bias-tee 107 treating the modulation signal to match it to the impedance of the modulator 101. With the above detailed description of this embodiment of the invention, the optical comb is obtained. The comb light is coupled out using an optical lens 103 and a coupling fiber 108. Some characteristics of the OFC generation system are shown in Figures 7-9.

Figure 6 is an embodiment of the QCPM phase modulator of this invention. This phase modulator is applied in the embodiment of the OFC generator defined by Fig.5. This is a MQW QCSE phase modulator with a vertical cavity structure, comprising 60/61 quaternary/quaternary InGaAsP/InGaAsP well/barrier region 20 with well width of lOnm per well and 7.5nm width per barrier, two InP spacers 21, 22 sandwiching the MQW region 20, a 16 pairs of InGaAsP/InP bottom DBR mirror stack with a reflectivity of 80% 30 next to the InP spacer 21, a p-type InP contact layer 31 on which, the central optical aperture 32 is 60μm in diameter and anti-reflection coated with Si N₄ material deposited using plasma enhanced chemical vapour deposition (PECVD), a n-type InP layer 23 on which, all above described layers are grown using metallo-organic vapour phase epitaxy, a separated n type InP layer 24 by etched isolation gap 29, a semi-insulating InP substrate 25 on which, all above described layers are grown, a metal n contact on the layer 23, a pad 28 and a metal air-bridge p contact 26. The QCSE tuning element with MQW is an etched mesa of 400μm in diameter and fabricated using standard photolithography, thermal evaporation and wet chemical etching. The described preferred embodiment of the QCSE phase modulator is a fast modulator that results in a 3dB cut-off frequency of over 6GHz due to application of air-bridged p-type contact which gives rise to a smaller electrical contact area. 7. REFERENCES

[1]. M. Zirngibl. C. H. Joyner, C. R. Doerr, L. W. Stulz and H. M. Presby, "An 18 channel multifrequency laser " IEEE Photonics Technology Letters, vol.8, no.7, pp.870-872, 1996.

[2]. M. Kourogi, T. Enami and M. Ohtsu, "Coupled-cavity monolithic optical frequency comb generator", IEEE Photonics Technology Letters, vol.8, no.12, pρ.1698-1700, 1996

[3]. K. Shimizu, T. Horiguchi, and Y. Koyamada, "Broad-band absolute frequency synthesis of pulsed coherent lightwaves by use of a phase-modulation amplified optical ring", IEEE Journal of Quantum Electronics, vol.33, no.8, 1997

[4]. M. Kourogi, T. Saito, T. Enami and M. Ohtsu, "Monolithic optical frequency comb generayors at 1.5μm wavelength region", Conference Proceedings-LEOS Annual Meeting, Vol.8, pp.272-273, 1994 [5]. A.E. Siegman, "Lasers", University Science Books, pp.1095-1101, 1986

[6]. P.S. Spencer, D.M. Kane, K.A. Shore, "Transition to pulsed operation in short external-cavity FM semiconductor lasers", IEEE J. Quant. Electron., Vol.35, No.5, pp.788-793, 1999

[7]. K.A. Shore, D.M. Kane, "Cavity decoupling in external cavity FM diode lasers", Electron. Lett. Vol.33, No.l, ρp.50-51, 1997

[8]. A. P. Willis and D. M. Kane, "Modulation induced coherence collapse in FM diode lasers", Opt. Comm., vol.107, pp.65-70, 1994

[9]. CH. Henry, R.A. Logan, K.A. Bertness, "Spectral dependence of the change in refractive index due to carrier injection in GaAs lasers", J. Appl. Phys., Vol.52,

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Claims

WHAT IS CLAIMED IS:

1. An optical frequency comb generator, comprising: a quantum confined phase modulation (QCPM) component driven in use by an electrical modulating signal; a laser gain component coupled to said QCPM component; and an electrical modulating signal driving the QCPM where the modulating frequency sets the spacing of the optical comb spectrum.

2. A generator according to claim 1, wherein said QCPM modulation component is a semiconductor QCPM modulator.

3. A generator according to claim 1, wherein said laser gain component is one of laser and laser array components capable of coupling with the QCPM component to produce frequency modulation laser operation.

4. A generator according to claim 1, wherein the QCPM component and the laser gain component are so coupled to form a monolith.

5. A generator according to claim 1, wherein the generator is an external cavity optical frequency comb generation system.

6. A generator according to claim 1, wherein said QCPM component and the laser gain component are coupled directly or by lenses along the cavity axis of the generator.

7. A generator according to claim 4, wherein the monolith integrates the components on a single substrate to form either a Fabry-Perot or a ring cavity.

8. A generator according to claim 4, wherein the comb light is coupled out using one of the surfaces of the defined laser gain component and the QCPM component.

9. A generator according to claim 5, where the said external cavity comprises one of FP cavities or ring cavities.

10. A generator according to claim 5, wherein the comb light is coupled out from at least one optical mirror of the external cavity.

11. A generator according to claim 2, wherein the QCPM component comprises an active region of quantum wells, quantum wires, or quantum dots.

12. A generator according to claim 11, wherein the QCPM component is one of a waveguide QCPM modulator and a QCPM modulator with light propagating normal to the plane of the quantum wells, wires or dots.

13. A generator according to claim 1, wherein the QCPM component is a quantum confined stark effect phase modulator.

14. A generator according to claim 12, wherein the modulator has an air-bridged contact between the active area and the contact pad.

15. A generator according to claim 12, wherein the waveguide QCPM modulators includes cleaved end facets.

16. A generator according to claim 12, wherein the QCPM modulator with light propagating normal to the plane of the quantum wells, wires or dots includes an integrated reflecting mirror structure.