Effective optical path length compensable optical device

Details Effective optical path length compensable optical device Effective optical path length compensable optical device

1998-03-30

2002-01-08

Lee, John D. (Department: 2874)

Optical waveguides

Polarization without modulation

C385S014000, C385S040000, C385S130000

Reexamination Certificate

active

06337931

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an optical device to be suitably used for the reduction of inter-polarization errors in optical signals taking place in conjunction with the higher-speed oriented transmission of optical signals, for example, in terminal apparatus or repeating installations of optical communication systems.
In recent years, in accordance with the progress of advanced information-centered society, a demand arises for the transmission of a huge quantity of information, so that an optical communication system designed to transmit information in the form of optical signals has been put to practical use as a means for the transmission of such a huge quantity of information.
In the optical communication system, with the increase in signal modulation speed, approaches to higher-speed transmission have been taken year by year, and an optical device of an optical waveguide type having a double refraction (birefringence), such as an optical external modulator for accomplishing the high-speed modulation of signals, has been in use in every place.
In addition, recently, approaches to the use of the optical waveguide type device have been taken even in the field of measurements and others.
2) Description of the Related Art
So far, in the prior optical communication systems, the transmission speed (bit rate) of digital optical signals has mainly been below 1 Gb/s, and in the case of transmitting the optical signals at such a transmission speed, the length of a 1-bit optical pulse in an optical signal (the bit length of an optical signal=the velocity of light in a propagating medium×the light emission time per 1 bit) exceeds 30 cm.
Now, let it be assumed that an optical device is constructed with a lithium niobate (LiNbO
3
) substrate (board) having an extremely high double refraction. In this case, since the double-refractive index difference of the lithium niobate substrate due to the light polarization (TE mode and TM mode) in an optical signal is approximately 0.08, for instance, even in case where an optical signal propagates within an optical device having a length of approximately 40 mm, a bit error of approximately 3.2 mm only occurs between the polarization components of the optical signal. Further, such a bit error is as small as approximately 2% of the bit length of the optical signal even taking into consideration the double refraction of the lithium niobate substrate, and therefore, particularly attention has not been paid toward this bit error.
Besides, in the prior optical communication systems, the optical devices on which the double refraction has adverse influence have not been put to use.
Incidentally, in a transmission section of a terminal apparatus of an optical communication system, an optical modulator where an optical waveguide is made in a lithium niobate substrate has been employed as an optical device, and since this optical modulator deals with a single polarization, the above-mentioned bit error does not occur.
There is a problem which arises with the prior optical communication systems, however, in that, in the case that the optical signal transmission speed increases, difficulty is encountered to disregard the influence from the above-mentioned bit error.
For instance, in the case that the optical signal transmission speed reaches 10 Gb/s, the bit length of the optical signal comes to approximately 3 cm, and when the optical signal transmission speed reaches 40 Gb/s, the bit length of the optical signal comes to approximately 7.5 mm. If propagating the data with such a bit rate within the foregoing optical device, the information representative of the optical signal can undergo damages due to the influence from the bit error of approximately 3.2 mm.
Moreover, in recent years, in the optical communication system, in addition to the terminal apparatus, an optical device having a double refraction starts to be used even for repeating installations placed in transmission lines, and therefore, consideration should also be given to the effects from the accumulation of bit errors caused by this double refraction.
FIG. 11
is an illustration of one example of the countermeasures against the double refraction.
In
FIG. 11
, an optical device
100
is designed to make optical signals inputted from a plurality of input optical waveguides
101
a
interfere with each other to output the optical signals through desired output optical waveguides
101
f
, and functions as an array waveguide type diffraction grating.
Reference numeral
101
represents an optical waveguide assembly comprising the input optical waveguides
101
a
, a plane optical waveguide
101
b
, channel optical waveguides
101
c
,
101
d
, a plane optical waveguide
101
e
, and the output optical waveguides
101
f
. The input optical waveguides
101
a
and the output optical waveguides
101
f
, the plane optical waveguide
101
b
and the plane optical waveguide
101
e
, and the channel optical waveguides
101
c
and the channel optical waveguides
101
d
are formed into symmetric configurations, respectively.
Although this optical waveguide assembly
101
is constructed in a manner that a glass (SiO
2
) is melted on a silicon (Si) substrate at a high temperature, since stress occurs in the optical waveguide assembly
101
in the process of returning the temperature of the glass melted at the high temperature to the room temperature, the optical waveguide assembly
101
gets to show a double refraction.
When the optical waveguide assembly
101
thus has the double refraction, since a phase error takes place between the polarization components of an optical signal propagating within the optical waveguide assembly
101
, the respective polarization components of the optical signal exit from the different optical waveguides
101
f.
In addition, in this optical device
100
, a half-wave plate
102
for the conversion of the polarization condition of an optical signal is disposed at an middle position of the optical waveguide assembly
101
(between the channel optical waveguides
101
c
and the channel optical waveguides
101
d
) in a state of being inclined by 45 degrees with respect to the respective channel optical waveguides
101
c
and
101
d
, thereby rotating the polarization condition of the optical signal by 90 degrees so that the inter-polarization effective optical path lengths for the optical signal (polarization-separated optical signals) become equal to each other to offset the phase error between the polarization components due to the double refraction.
However, the phase error between the polarization components the optical device
100
shown in
FIG. 11
tries to reduce occurs because the phase of the light wave of the optical signal shifts, but essentially differing from the aforesaid bit error (which takes place because the bit itself of an optical signal shifts) occurring, in conjunction with the high speed transmission of optical signals, due to the double refraction of the optical device constructed using a lithium niobate substrate or the like.
That is, the phase error between the polarization components treated as a problem in
FIG. 11
assumes as a value as the order of the wavelength of an optical signal (as the case may be, several times the order of the wavelength), whereas the above-mentioned optical signal bit error reaches a large value above 1000 times the phase error.
In the case of an optical signal with as a very high speed as tera bit being inputted to the optical device
100
, although it may be considered that the optical device
100
can reduce the bit error of the optical signal because the bit length of the optical signal becomes approximately 150 &mgr;m (approximately {fraction (1/10)} of the order of the wavelength of the optical signal), in fact the optical device
100
is incapable of reducing the bit error of the optical signal.
This is because the optical device
100
originally exerts its function by successively shifting optical signals in the channel waveguides
101
c
,
101
d
and

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