Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2003-06-06
2004-10-19
Thompson, Timothy (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S285000, C359S287000, C385S122000
Reexamination Certificate
active
06806986
ABSTRACT:
This application claims priority from Japanese Patent Application Nos. 2002-174938 filed Jun. 14, 2002 and 2003-020560 filed Jan. 29, 2003, which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength converter and a wavelength converting apparatus, and more particularly to a wavelength converter and a pump wavelength variable type wavelength converting apparatus that can be designed to handle a given number of pump wavelengths, can prevent reduction in a conversion efficiency, and can be simply configured using a practical size nonlinear optical material.
2. Description of the Related Art
Conventionally, wavelength converters and wavelength converting apparatuses configured using them have been known utilizing a variety of second order nonlinear optical effects. For example, a second harmonic generation apparatus can convert incident light to light (second harmonics) with half the original wavelength (twice the frequency). A sum frequency generation apparatus can convert two light beams with different wavelengths into a light beam with a frequency corresponding to the sum frequency of the two frequencies.
On the other hand, difference frequency generation apparatus can convert two light beams with different wavelengths into a light beam with a frequency corresponding to the difference frequency between the two frequencies. In addition, when one of the incident light beams is larger enough than the other of them, it can be configured as an optical amplifier that amplifies the intensity of the incident light utilizing a parametric effect. It is also applicable as a wavelength tunable light source by configuring a parametric resonator utilizing the parametric effect.
Next, the operation principle of conventional wavelength converters will be described briefly by way of example of difference frequency generators utilizing the second order nonlinear optical effect. These converters convert signal light with a wavelength &lgr;
1
to idler light with a wavelength &lgr;
2
by launching the signal light to a nonlinear optical medium pumped by pump light with a wavelength &lgr;
3
. The following equation is capable of coping with the three wavelengths, including the case where &lgr;
1
=&lgr;
2
.
1
λ
3
=
1
λ
1
+
1
λ
2
(
1
)
Research and development of various materials have been conducted as nonlinear optical media capable of coping with such elements. As for element structures, the so-called “quasi-phase match type structure” is considered to be promising as reported by M. H. Chou, et al., (Optics Letters, Vol. 23, p. 1004 (1998)), for example. It has a structure that causes a second order nonlinear optical material such as LiNbO
3
to vary (modulate) its nonlinear optical coefficient periodically at a uniform period.
FIGS. 1A and 1B
are diagrams for explaining a conventional wavelength converter (difference frequency generator) utilizing the second order nonlinear optical effect:
FIG. 1A
is a diagram illustrating a configuration of the wavelength converter conceptually; and
FIG. 1B
is a diagram illustrating the dependence of a conversion efficiency on a phase mismatch amount. To create a quasi-phase match type structure in a second order nonlinear optical material, the following methods are conceivable: First, a method of carrying out periodical modulation by spatially, alternately reversing the sign of the nonlinear optical coefficient of the material; second, a method of carrying out the periodical modulation by alternately placing sections with large and small nonlinear optical coefficients.
As for a ferroelectric crystal such as LiNbO
3
, the sign of the nonlinear optical coefficient (d constant) corresponds to the polarity of the spontaneous polarization. Thus, in the wavelength converter shown in
FIG. 1A
, an optical waveguide
12
is formed in a nonlinear optical medium, a LiNbO
3
substrate
11
, by a proton exchange method to periodically reversing the spontaneous polarization of the LiNbO
3
at a modulation period (modulation period of the nonlinear optical coefficient) &Lgr;
0
=14.75 &mgr;m, thereby modulating the nonlinear optical coefficient. The wavelength converter is supplied with signal light
13
and pump light
15
via a multiplexer
17
. The wavelength converter can carry out the wavelength conversion of the 1.55 &mgr;m band signal light
13
by the 0.78 &mgr;m band pump light
15
.
In such a converter, the phase mismatch amount &Dgr;&bgr; is given by the following equation.
Δ
β
=
2
π
·
(
n
3
λ
3
-
n
2
λ
2
-
n
1
λ
1
)
(
2
)
where n
1
is the refractive index of the LiNbO
3
for the signal light
13
with the wavelength &lgr;
1
; n
2
is the refractive index for idler light (difference frequency light)
14
with the wavelength &lgr;
2
; n
3
is the refractive index for the pump light
15
with the wavelength &lgr;
3
; and &Lgr;
0
is the modulation period of the nonlinear optical coefficient. The conversion efficiency &eegr; is given by the following equation using the phase mismatch amount &Dgr;&bgr;.
η
=
η
max
·
{
sin
[
(
Δ
β
-
2
π
Λ
0
)
·
L
2
]
[
(
Δ
β
-
2
π
Λ
0
)
·
L
2
]
}
2
(
3
)
where L is the length of the nonlinear optical medium in the waveguide direction. Accordingly, the conversion efficiency &eegr; of the wavelength converter takes the maximum value when the phase mismatch amount &Dgr;&bgr; is 2&pgr;/&Lgr;
0
. For example, consider the case where the wavelength &lgr;
1
of the signal light
13
is fixed. In this case, the wavelength of the pump light
15
that satisfies the “quasi-phase matching condition”, in which the phase mismatch amount &Dgr;
62
given by the foregoing equation (2) becomes 2&pgr;/&Lgr;
0
, depends on the chromatic dispersion of the refractive index of the nonlinear optical medium, and is determined uniquely if the modulation period &Lgr;
0
is given.
Varying the wavelength of the pump light
15
from the quasi-phase match wavelength that satisfies the quasi-phase matching condition, the conversion efficiency &eegr; reduces according to the foregoing equations (2) and (3).
FIG. 1B
is a graph illustrating the dependence of the conversion efficiency &eegr; on the phase mismatch amount &Dgr;&bgr; in which the conversion efficiency &eegr; is normalized in such a manner that the maximum value becomes one. Assume that the length of the optical waveguide
12
of the wavelength converter consisting of the LiNbO
3
is 42 mm. Then, the band of the phase mismatch amount &Dgr;&bgr; in which the conversion efficiency &eegr; becomes half the maximum value is very narrow of about 0.1 nm in terms of 0.78 &mgr;m band pump wavelength.
As is clear from the foregoing equation (1), a plurality of pump light beams with different wavelengths are required to convert the wavelength &lgr;
1
of the signal light
13
to the difference frequency light with a given wavelength (&lgr;
2
′) However, the conventional modulation structure as illustrated in
FIG. 1A
, in which the nonlinear optical coefficient varies periodically at a uniform period, cannot vary the wavelength of the pump light substantially because of the narrow allowable range of the wavelength of the pump light. As a result, it cannot achieve the conversion to the difference frequency light with a given wavelength.
Next, to handle the different pump light wavelengths, a method is also possible in which modulation structures with a plurality of different modulation periods are disposed sequentially in the longitudinal direction. However, when the total length of the nonlinear optical media is fixed, the length of a nonlinear optical medium used in each modulation period is reduced. Generally, the conversion efficiency &eegr; of the wavelength converter utilizing the second order nonlinear optical effect is proportional to the square of the length of the nonlinear optical
Asobe Masaki
Miyazawa Hiroshi
Nishida Yoshiki
Suzuki Hiroyuki
Tadanaga Osamu
Nippon Telegraph and Telephone Corporation
Thompson Timothy
Workman Nydegger
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