Optical waveguides – Accessories – Attenuator
Reexamination Certificate
2002-07-17
2004-11-16
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Accessories
Attenuator
C385S014000, C385S045000
Reexamination Certificate
active
06819859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a planar lightwave circuit type variable optical attenuator that utilizes a Mach-Zehnder interferometer and adjusts an optical length with a thin-film heater.
2. Description of the Related Art
In Wavelength Division Multiplexing (WDM) optical communication, optical signals emitted from a plurality of light sources and having different wavelengths in the band of 1.55 &mgr;m are transmitted through a single optical fiber transmission line and received by light receivers allocated for each wavelength, thereby realizing large-capacity optical communication. Respective powers of the optical signals at the light receivers are required to be equal to one another. A variable optical attenuator is necessary to meet that requirement.
As variable optical attenuators, there are known a planar lightwave circuit type utilizing a Mach-Zehnder interferometer and adjusting an optical length with a thin-film heater, a bulk type optical system in which an optical filter is driven by a stepping motor, and a bulk type optical system utilizing a Faraday rotator. Of those variable optical attenuators, the planar lightwave circuit type has many advantages over the other types: it can be downsized through integration and is superior in productivity for mass production, and it has smaller loss.
FIG. 1
shows a conventional planar lightwave circuit type variable optical attenuator
1
. The illustrated variable optical attenuator
1
includes a Mach-Zehnder interferometer
101
comprising two optical waveguide arms
10
,
20
buried in a clad
70
formed on a substrate
100
, and two directional couplers
50
,
51
for coupling the optical waveguide arms
10
,
20
together at their both end. A thin-film heater
110
is disposed, extending along the optical waveguide arm
10
, on the surface of the clad
70
. In
FIG. 1
, “I
in
” represents an input optical power, and “I
out
” represents an output optical power.
An electric power W supplied to the thin-film heater
110
is controlled by a control unit
200
for adjusting the temperature of the thin-film heater
110
. Upon heating of the thin-film heater
110
, the temperature of the optical waveguide arm
10
rises and the refractive index of the optical waveguide arm
10
changes with the thermooptic effect. As a result, a difference in optical length between the optical waveguide arms
10
and
20
is also changed, whereby the output optical power I
out
can be varied according to the optical interference.
A difference &Dgr;L
0
between the arm lengths under a condition in which no electric power is supplied to the thin-film heater
110
, is set to zero or &lgr;
0
/2n
eff
(&lgr;
0
is the central wavelength of an operating wavelength band and n
eff
is the effective refractive index of the optical waveguide) corresponding to a phase difference 180°. When the arm length difference &Dgr;L
0
is zero, the input optical power is output to an output port positioned in a diagonal relation to an input port. When the arm length difference &Dgr;L
0
is &lgr;
0
/2n
eff
, the input optical power is output to the output port positioned on the same side as the input port.
When the thin-film heater is disposed on the clad, the optical waveguide arm is susceptible to thermal stress in the vertical direction because of a difference in thermal expansion coefficient between the core and the clad. The thermal stress generates double refraction in the optical waveguide arm because of photoelasticity and hence increases polarization dependence of optical characteristics. In the optical attenuator of
FIG. 1
, for example, as the heating temperature of the thin-film heater
110
rises, a larger attenuation is obtained, but a Polarization Dependent Loss (PDL) is also increased at the same time.
With respect to such problem, a reference “Proceedings of the 2001 Institute of Electronics, Information and Communication Engineers General Conference, C-3-64, p. 229, Lower PDL of PLC Type Variable Attenuator” describes a technique for reducing the PDL by means of thermal stress releasing grooves formed on both sides of an optical waveguide arm.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a planar lightwave circuit type variable optical attenuator in which a small PDL is maintained even when attenuation is large.
To achieve the above object, the present invention provides a planar lightwave circuit type variable optical attenuator including a Mach-Zehnder interferometer formed on a substrate. The Mach-Zehnder interferometer comprises two optical waveguide arms formed on the substrate, and thin-film heaters formed respectively over the two optical waveguide arms, the two optical waveguide arms having lengths not equal to each other.
Assuming that an effective refractive index of the optical waveguide arms is n
eff
and a central wavelength of an operating wavelength band is &lgr;
0
, a difference &Dgr;L
0
between the lengths of the two optical waveguide arms may satisfy the following relationship:
0.36×&lgr;
0
eff
≦&Dgr;L
0
≦0.47×&lgr;
0
eff
An attenuation may be in the range of 7 to 21 dB when no electric powers are supplied to the thin-film heaters for adjusting the optical lengths of the optical waveguide arms. Plural sets of Mach-Zehnder interferometers may be formed on the substrate in parallel.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.
REFERENCES:
patent: 6351581 (2002-02-01), Doerr et al.
patent: 2003/0180027 (2003-09-01), Oaknin et al.
“Proceedings of the 2001 Institute of Electronics”, Information and Communication Engineers General Conference, C-3-64, p. 229, Lower PDL of PLC Type Variable Attenuator. (Prior Art cited in the Specification. We believe that it is not necessary to prepare an English translation of this document because the contents are discussed in the specification and related to the same technique as in the following document.)
“Polarization Sensitivity of a Silica Waveguide Thermooptic Phase Shifter for Planar Lithtwave Circuits”, IEEE Photonics Technology Letters, vol. 4, No. 1, Jan. 1992, pp. 36-38.
Hatayama Hitoshi
Hirose Chisai
Palmer Phan T. H.
Sumitomo Electric Industries Ltd.
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