A transmitter optical sub-assembly (TOSA) structure having an independent upward heat dissipation path for dissipating heat in an upward direction including an independent signal source, an LDU assembly including a laser diode emitting a plurality of optical signals, a cascade LDU holding the laser diode, a lens positioned in front of the laser diode on the cascade LDU and an optical bench assembly including an optical bench assembled on a photonic integrated circuit having a plurality of passive optical components assembled on the optical bench. In particular, the independent signal source, the laser diode and the cascade LDU, are independent from the plurality of passive optical components on the photonic integrated circuit.