Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
2001-11-08
2004-10-05
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C359S326000, C359S328000
Reexamination Certificate
active
06801356
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to optical devices based on parametric processes in non-linear waveguides.
In a non-linear process involving multiple interacting fields of different frequencies, the phase velocities at the different frequencies will usually differ. As a result there will be no significant net energy transfer between the different interacting fields unless measures are taken to provide overall phase matching. Phase matching requires that the relative phase mismatch between the interacting fields is zero over the length of the optical path. Phase matching can be achieved in several ways, of which so-called quasi phase matching (QPM) is one widely used technique [1].
QPM is based on the idea of providing a periodic modulation of the non-linear susceptibility &khgr; along the optical path of a non-linear material, with the periodic modulation having a period &agr; matched to the length over which two interacting fields develop a relative phase mismatch of half a period, &pgr;. In fact, QPM does not address the phase mismatch locally, but provides overall compensation between regions of differing non-linearity, so that efficient net energy transfer from a pumping wave to a signal wave can take place.
For a second order non-linear (SON) parametric down conversion (PDC) process of the kind &ohgr;
p
=&ohgr;
s
+&ohgr;
i
where &ohgr;
p
. &ohgr;
i
and &ohgr;
i
are the pump, signal and idler frequencies, respectively, the phase matching (PM) condition can be expressed as:
&Dgr;&bgr;=&bgr;(&ohgr;
p
)−(&ohgr;
s
)−&bgr;(&ohgr;
i
)=0
where &bgr;(&ohgr;
p
) &bgr;(&ohgr;
s
) and &bgr;(&ohgr;
i
) are the propagation constants of the pump, signal and idler waves, respectively.
Other expressions hold for different non-linear processes, such as third harmonic generation, four-wave mixing etc. For example, for second harmonic generation (SHG) in which a pump beam is used to generate a frequency-doubled signal, the PM condition can be expressed as &Dgr;&bgr;=&bgr;(2&ohgr;)−2&bgr;(&ohgr;)0.
It is mentioned that the SHG and PDC processes may be considered to be the reverse of each other. In the low conversion regime, in which the undepleted pump approximation holds, SH efficiency and the parametric gain assume similar expressions [3, 12]. Moreover, the bandwidth BW for SHG corresponds to the bandwidth BW for PDC, when they are read in terms of SH wavelength and pump wavelength respectively. However, the bandwidth BW for the PDC process for signal and idler wavelengths (for a fixed pump wavelength) is much broader, in particular at degeneracy [3, 12]. In the following, bandwidth is discussed in terms of SHG, although the findings can be extended to the pump bandwidth in a PDC process.
Returning to the specific example of the SON-PDC process, quasi phase matching includes an additional grating contribution term m(27c/&Lgr;) which arises from the periodic modulation for the m-th order harmonic, assuming a square-wave modulation of the non-linear properties. The QPM condition is then:
&Dgr;&bgr;=(&ohgr;
p
)−&bgr;(&ohgr;
s
)−&bgr;(&ohgr;
i
)−
m
(2&pgr;/&Lgr;)=0
It is also known that the efficiency &eegr; of a parametric process in the low conversion efficiency regime is given by the expression:
&eegr;∝
L
2
sinc
2
(&Dgr;&bgr;
L/
2)
where L is the interaction length. Thus, efficiency can be improved by increasing the interaction length L (where &Dgr;&bgr;~0) or reducing &Dgr;&bgr;, i.e. by better satisfying the QPM condition. For example, it will be possible to determine for a given non-linear material the variation of efficiency of a non-linear process as a function of signal and/or pump frequency, efficiency being maximized for the set of wavelengths which satisfy the QPM condition.
The usable bandwidth for a non-linear process can be defined in terms of a threshold of sinc
2
(&Dgr;&bgr;L/2), such as:
sinc
2
(&Dgr;&bgr;
L/
2)≧½
which can be rewritten as:
&Dgr;&bgr;≦0.886&pgr;/L
from which it is apparent that, while a long interaction length L improves efficiency, it narrows the bandwidth. The only way of improving efficiency without sacrificing bandwidth is thus by improving the phase matching, i.e. by reducing &Dgr;&bgr;.
The design of a waveguide structure for parametric processes has to consider several parameters, including efficiency, bandwidth, and single mode operation. In the low conversion regime, and assuming a uniform non-linearity across the beam profile, the SHG efficiency (or parametric gain in PDC) is of the form
η
∝
L
2
A
ovl
sin
c
2
(
Δβ
L
2
)
(
1
)
where A
ovl
is the effective area which takes account of the overlap of the interacting waves: A
ovl
=1/I
ovl
2
where the overlap intensity I
ovl
=|∫E
SH
*
E
F
2
dA | with E
SH
and E
F
the normalised second harmonic (SH) and fundamental transverse profile respectively-defined so that ∫|E
SH
|
2
dA=∫|E
F
|
2
dA=1. The other parameters of the expression have already been defined. The effect of the factors &Dgr;&bgr; and L on efficiency &eegr; has already been discussed. From this equation it is however also apparent that efficiency &eegr; can be increased by reduction of the effective overlap area A
ovl
in absolute units of area (and increase of the degree of overlap)
Having now described the basic design considerations for designing devices based on non-linear effects, the prior art is reviewed.
Efficient QPM-SHG and PDC have been demonstrated in periodically poled ferroelectrics waveguides [2a, 2b] and glass fibers [3, 12]. Through the use of considerable interaction lengths L, good efficiency has been achieved in SHG devices, at the cost of limited bandwidth BW and lack of single mode operation at all wavelengths.
These prior art periodically poled ferroelectrics waveguides and glass fibers thus cannot deliver the large bandwidth that is important to ensure stable operation of SHG devices (for example temperature stability when high power is involved) or for SHG devices operating in the pulsed regime where one requires that interaction occurs for all the spectral components of the pulse.
Moreover, in the case of PDC (or difference frequency generation) a broad pump wavelength bandwidth is crucial for certain applications, for example in the routing of a multi-channel WDM system when one wants to have the possibility to switch from any channel into any other channel using difference frequency generation. The fact that typically one has large signal and idler bandwidths and narrow pump bandwidths means that routing around the degenerate point is possible, but not from one channel close to this point to another far from it (this would imply the use of other pump wavelengths). Basically, the device would function as an efficient spectral inverter [4].
To enhance the product of bandwidth and interaction length, BW•L, it has been proposed to use aperiodic QPM structures [5]. The resulting efficiency is however small compared to that achievable for the same device length with a periodic QPM structure due to a reduction of the effective interaction length L.
As far as single mode operation in QPM-PDC (and difference frequency generation) devices is concerned, it has been proposed that complex waveguide structures [6] can be used to launch efficiently the pump at short wavelength into a waveguide which is single mode for the idler and the signal frequencies. For example, the signal and idler may be generated at around 1.5 &mgr;m from a 0.75 &mgr;m pump in a waveguide with a first cut-off wavelength at 1.3 &mgr;m. The pump needs to be launched into the mode that satisfies the QPM condition at both the fundamental signal mode frequency and the idler mode frequency. Usually, the mode at the pump frequency also has to be the fundamental mode for efficient operation, i.e. goo
Broderick Neil
Monro Tanya
Pruneri Valerio
Richardson David
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lee John D.
University of Southampton
LandOfFree
Optical parametric devices and methods for making same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical parametric devices and methods for making same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical parametric devices and methods for making same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3323775