Reducing output noise in a ballast-powered semiconductor optical amplifier

A monolithic integrated semiconductor optical amplifier (SOA) for amplifying an optical signal, the SOA comprising:

• a signal waveguide extending between an input facet and an output facet and defining an optical signal path therebetween;
• and
• a plurality of ballast lasers extending across said optical signal path, each ballast laser defining a distinct amplification region where vertically coincident with said signal waveguide, each ballast laser being positioned with respect to said signal waveguide in said amplification region such that the optical signal is amplified therein using energy provided by said ballast laser, said plurality of ballast lasers including a first ballast laser and a second ballast laser, said first ballast laser being positioned nearer to said input facet than said second ballast laser;
• wherein said first ballast laser is configured to cause a greater optical signal gain per unit distance in its associated amplification region than said second ballast laser.

A method of operating a monolithic, integrated, ballast-powered semiconductor optical amplifier (SOA), the SOA having a signal waveguide defining an optical signal path between an input and an output, the SOA further comprising “N” amplification stages formed by “N” ballast lasers positioned across the optical signal path, each ballast laser comprising multiple electrical segments including a main segment that powers the amplification stage and an auxiliary segment, comprising:

• receiving a signal modulation rate F, a desired overall gain G, and a desired output saturation power PSAT;
• assigning a gain profile g1, g2, . . . , gN for successive amplifier stages whose combination achieves the desired overall gain G;
• computing a set of minimum amplification stage output saturation powers PSAT1, PSAT2, . . . , PSATN corresponding to said desired output saturation power PSAT and to said gain profile g1, g2, . . . , gN;
• for each ballast laser “i,” determining an auxiliary segment bias current IAUXi that achieves the desired amplifier stage gain gi;
• for each ballast laser “i,” determining a main segment output saturation threshold current IMAINSATTHRESHi by determining the main segment excitation current IMAINi necessary to result in a stage output saturation power of PSATi;
• for each ballast laser “i,” determining a main segment relative intensity noise (RIN) threshold current IMAINRINTHRESHi by determining the main segment excitation current IMAINi necessary to result in a relaxation oscillation frequency (ROF) greater than the signal modulation rate F such that a RIN spectrum value at the modulation rate F is equal to a predetermined tolerance amount above a low-frequency noise floor of the RIN spectrum;
• supplying each ballast laser “i” with said auxiliary segment threshold current of IAUXi;
• and
• supplying each ballast laser “i” with a main segment threshold current that is not less than the greater of (i) said main segment output saturation threshold current IMAINSATTHRESHi and (ii) said main segment RIN threshold current IMAINRINTHRESHi.