Polarization-insensitive optical modulators

Details Polarization-insensitive optical modulators Polarization-insensitive optical modulators

1995-08-03

2001-08-14

Lester, Evelyn A (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S250000, C257S018000, C257S021000, C257S184000, C257S189000, C257S190000, C257S191000

Reexamination Certificate

active

06275321

ABSTRACT:

BACKGROUND TO THE INVENTION
Since it became clear that the wavelength chirp present with direct modulation of a laser source would severely limit the possible modulation band-widths available for long-wavelength (around 1.5 &mgr;m) optical communication, there has been great interest in developing high-speed electroabsorption and electroreflectance modulators. This has led to the design of a number of modulator and integrated laser-plus-modulator structures based on the quantum confined Stark effect (QCSE). The quantum well layers of such quantum well semiconductor structures are typically single composition layers possessing no substructure, and hence providing wells that are substantially flat-bottomed and square-sided. There has, however, also been some investigation of the properties of quantum well layers that do possess a substructure, such a substructure being provided to produce a more complicated well profile. Thus EP 0 416 879 proposes a QCSE modulator whose quantum well layers have a substructure comprising two substructure layers of different material designed to provide the well with a built-in step that produces a dipole at zero applied electric field, and in consequence is designed to produce applied field induced band edge shifts without any significant change in absorption coefficient. A somewhat similar structure is described by T. Tütken et al in a paper entitled, “Large observed exciton shifts with electric field in InGaAs/InGaAsP stepped quantum wells”, App. Phys. Lett. 63 (8), Aug. 22, 1993, pp 1086-1091, whose authors are concerned to maximise the QCSE shift for a given value of applied field. The effects upon various parameters, including Stark shift, exciton binding energy, and overlap of electron and hole wavefunctions, produced by changing the shape of quantum wells, have been made the subject of study in a paper by W. Chen & T. G. Anderson entitled “Quantum-confined Stark shift for differently shaped quantum wells”, Semicond. Sci. Technical 7 (1992) pp 823-836. This paper treats not only substructures providing 2-step and 3-step wells, but also wells with profiles that are partly or wholly continuously graded. A quantum well substructure can also be employed to provide a QCSE device that exhibits a blue-shift of absorption edge instead of the red-shift exhibited by QCSE devices of more conventional type. Such a blue-shift device is described by P. N. Stavrinou et al in a paper entitled, “Use of a three-layer quantum-well structure to achieve absorption edge blueshift”, App. Phys. Lett. 64 (10), Mar. 7, 1994, pp 1251-1253. Blue-shift resulting from a non-uniform composition quantum well is also described by W. Zhou et. al. in a paper entitled, “Simultaneous blue- and red-shift of light-hole and heavy-hole band in a novel variable-strain-quantum well heterostructure”, App. Phys. Lett. 66 (5) Jan. 30, 1995 pp 607-609. This describes a strained quantum well with a graded composition providing a value of strain that is graded in magnitude from one side of the well to the other. The paper explains that this grading makes the device polarisation controllable, providing it with the property that, at a unique value of applied bias, there is a cross-over between the red- and blue- Stark shifts for the heavy- and light-hole transitions.
In an integrated laser-plus-modulator the state of polarisation of the laser light that is incident upon the modulator is fully determined, and this may also be conveniently arranged to be the situation in the case of laser-plus-modulator configurations that are not integrated. Under these circumstances matters can usually be arranged so that any polarisation sensitivity exhibited by the modulator is of no practical consequence. On the other hand, there are other applications of modulator structures, such as pulse train shaping, where the polarisation of the light beam is unknown. There have therefore been attempts to design polarisation-insensitive quantum confined Stark effect (QCSE) modulators for these ‘between fiber’ applications. In the long wavelength range (around 1.5 &mgr;m), studies have been made of InP-based devices containing InGaAs layers with a modest amount of tensile strain (less than 1% lattice mis-match). With the appropriate strain for the well thickness, the E
1
-HH
1
transition (which is responsible for most of the TE absorption) and E
1
-LH
1
transition (which is responsible for TM absorption) can be made degenerate at zero bias. This has been shown in the paper by T. Aizawa, K. G. Ravikumar, S. Suzaki, T. Watanabe, and R. Yamauchi, “Polarisation-independent quantum confined Stark effect in an InGaAs/InP tensile-strained quantum well”, IEEE Journal of Quantum Electronics, 1994, 30, pp. 585-593, to correspond to equal TE and TM absorption of the incident light beam at zero bias, and hence to polarisation-insensitivity. However, as a field is applied across these structures the E
1
-LH
1
and E
1
-HH
1
transitions show different Stark shifts and do not remain matched because the shifts depend on the effective masses. As a result, existing modulator structures based on the QCSE are only truly polarisation insensitive at one field strength.
SUMMARY OF THE INVENTION
The present invention is directed to the design of a QCSE-based modulator which is substantially polarisation insensitive over a range of field strengths.
According to the present invention there is provided a quantum confined Stark effect modulator in which the or each quantum well layer of the modulator has a non-uniform composition that provides, across the thickness of the layer, a non-uniform value of lattice constant to produce a strain profile in the modulator that provides the modulator with substantially matching E
1
-HH
1
and E
1
-LH
1
Stark shifts for at least one polarity of applied electric field from 0 up to 100 kV/cm.


REFERENCES:
patent: 5090790 (1992-02-01), Zucker
patent: 5153687 (1992-10-01), Ishikawa
patent: 0416879A1 (1991-03-01), None
Chen, “Quantum-confined Stark shift for differently shaped quantum wells”, Semiconductor Sci. Technol., vol. 7, 1992, pp. 828-836.
Stavrinou, “Use of a three-layer quantum-well structure to achieve an absorption edge blueshift”, Applied Physics Letters, vol. 64, No. 10, Mar. 7, 1994, pp. 1251-1253.
Aizawa, “Polarization-Independent Quantum-Confined Stark Effect in an InGaAs/InP Tensile-Strained Quantum Well”, IEEE Journal of Quantum Electronics, vol. 30, No. 2, Feb. 2, 1994, pp. 585-592.
Zhou, “Simultaneous blue- and red-shift of light-hole and heavy-hole band in a novel variable-strain quantum well heterostructure”, Applied Physics Letters, vol. 66, No. 5, Jan. 30, 1995, pp. 607-609.
Tutken, “Large observed exciton shifts with electric field in InGaAs/InGaAsP stepped quantum wells”, Applied Physics Letters, vol. 63, No. 8, Aug. 23, 1993, pp. 1086-1088.

Affiliated with

Also associated with

Rating

Polarization-insensitive optical modulators does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polarization-insensitive optical modulators, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarization-insensitive optical modulators will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2439188