Optical modulator driver circuit with low power dissipation

Details Optical modulator driver circuit with low power dissipation Optical modulator driver circuit with low power dissipation

2002-12-11

2004-03-16

Dang, Hung (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S246000, C359S248000, C345S098000, C345S100000, C345S206000

Reexamination Certificate

active

06707589

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to modulator driver circuits and more particularly to an optical modulator with a driver output stage with low power dissipation.
2. Description of the Related Art
It is common practice in a drive circuit to employ transistor circuitry to provide a signal driver for modulating an electro-optical modulator, such as a Mach Zehnder modulator (MZM) or a semiconductor electro-absorption modulator (EAM). A typical driver circuit for an EAM is shown on U.S. Pat. No. 5,706,117 in FIG. 15, for example, as well as U.S. Pat. No. 6,014,392. The latter mentioned patent deals with wavelength dispersion of the EAM which is also discussed in U.S. Pat. No. 5,917,637. The output stage of such an optical modulator driver as known in the art is also illustrated in FIG.
1
. In
FIG. 1
, the signal source shown as driver amplifier
22
controls the peak to peak amplitude of the transition bits representing the signal bits. The drive circuit
10
employs a differential transistor pair arrangement
11
, also referred to as an emitter coupled differential circuit to provide current switching operation, which steers or directs the DC current to or away from optical modulator
12
. Modulator
12
is shown here as a EAM. Because an optical modulator responds to voltage, and not to current, the drive current must be converted to a voltage. A load resistor is added in parallel with modulator
12
, as shown in U.S. Pat. No. 5,706,117, to convert the drive current into a voltage. The power dissipation of circuit
10
is equal to the amount of the DC current, I
D
, in drive circuit 10 times the DC supply voltage, V
D
.
In the case of
FIG. 1
, there are two resistors
14
and
16
that are necessary to terminate the differential circuit. Only the voltage across resistor
14
is employed to drive modulator
12
. This impedance across or in parallel with modulator
12
is generally designed to be a 50 ohm load, as indicated in
FIG. 1
, from which impedance matching is achieved relative to RF transmission lines
18
and
20
. The resistance of resistors
14
and
16
is usually low, such as on the order of 50 &OHgr;, in order to match its resistance to the low impedance RF transmission lines
18
and
20
which electrically couple the RF drive signal from the output driver stage
10
to EAM
12
. The resistance of resistor
14
is also usually low in order that the modulator time constant, determined by the product of the optical modulator parasitic capacitance and its resistance, is low. As a result, the drive current, I
D
, will be high resulting in high power dissipation. As an example, for 2.5 V amplitude and 25 ohm load, the current could be 100 mA, for a supply voltage, V
DD
, about 5.2 V. This type of drive circuit
10
nominally consumes, for example, approximately 0.5 W of power.
One alternative approach is shown in the article of Kai-Yap Toh et al. entitled, “A 23-ps/2.1 mW ECL Gate with an AC-Coupled Active Pull-Down Emitter-Follower Stage”,
IEEE Journal of Solid
-
State Circuits
(Vol. 4(5), pp. 1301-1306, October, 1989. See, for example, the circuit in FIG. 5 of this article which comprises an emitter-coupled logic (ECL) circuit consisting of an emitter coupled differential circuit in combination with a “totem pole” circuit comprising a coupled emitter-follower pull-up transistor and a coupled emitter follower pull-down transistor, including a bias current circuit for the node between the control capacitor and the base of the pull-down transistor. A substantially same circuit is also shown in FIG. 13 and described in U.S. Pat. No. 5,574,391, which patent represents an improved upon circuit over the FIG. 13 ECL circuit by eliminating the bias current circuit to provide for even further reduced power consumption through the utilization of current mirror transistor circuit arrangement.
Still, the power consumption is higher than desired for many applications, particularly the application of interest here which is the modulation of optical modulators. In this connection, none of the foregoing emitter-coupled logic circuits deal with the issue of the deployment of such drive circuits in an optical modulator environment, such as a semiconductor EAM that requires high frequency modulation, such as 10 GHz or 40 GHz signals, to be applied via RF transmissions
18
and
20
. Another example of a totem pole bootstrap driver circuit used in telecommunications is illustrated in U.S. Pat. No. 6,002,269.
It is desirable that such high power dissipation in such drive circuits for high speed optical modulators be significantly reduced without the need for an RF transmission line and additional power consumption due to utilization of a pull-down transistor bias current circuit. This becomes particularly so where these drive circuits become integrated into packages containing photonic integrated circuit (PIC) chips, or even integrated as part of such a PIC chip in order that power and heat consumption can be minimized reducing both the overall power and dissipated heat budget of such packages and chips.
OBJECTS OF THE INVENTION
Therefore, it is an object of the present invention to overcome the aforementioned problem.
SUMMARY OF THE INVENTION
According to this invention, an optical modulator drive circuit provides a different approach to driving a optical modulator than presently employed, resulting in a reduction in power dissipation of the drive circuit by as much as about 80%. Therefore, this invention dissipates as little as 20% of power of the present drive circuits known in the art. The optical modulator may be a semiconductor electro-absorption modulator but the principle of the invention can be applied any other type of electro-optic modulator that relies on a voltage to modulate an optical signal.
The optical modulator is driven by an ECL circuit deployed in a “totem pole” or push-pull arrangement to apply a low-impedance voltage source to the optical modulator. No resistor is necessary in parallel with the modulator to provide the voltage to the optical modulator. The invention provides a relatively small current to charge and discharge the modulator capacitance, and is provided only during the bit transitions in the modulation of the signal. Outside of the modulation bit transitions, the current is automatically reduced even further. As a result, the output stage drive circuit nominally consumes much lower power by as much as 80%, which is approximately 20% of the prior art approaches, for example, power consumption in the range of about 0.1 W to about 0.20 W of power. This is of high importance where these circuits are deployed in connection with optical transmitter photonic integrated circuits (TxPICs) as illustrated in U.S. patent applications, Ser. No. 10/370,345 and 10/267,331, both filed on Oct. 8, 2002, which applications are incorporated herein by their reference. In this kind of optical integrated chip environment where there is an array of modulators integrated in a single semiconductor chip with a laser diode (LD) array, such as a DFB array, and an optical multiplexer, such as an AWG, power budget constraints become highly critical in the operation of the chip so that power consumption must be reduced to as low as possible without sacrificing high speed modulation such as at 10 GHz and 40 GHz.


REFERENCES:
patent: 6192167 (2001-02-01), Kissa et al.
patent: 6590691 (2003-07-01), Nagra et al.

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