Naoki Tsukiji

Challenges of Raman Amplification

Raman amplifiers are often regarded as a typical example of technologies rapidly developed in the midst of turmoil created by the so-called wavelength division multiplexing (WDM) bubble. Indeed, Raman amplifiers turned out to be technically very attractive in all the aspects of capacity, reach, and bit rate. Even though Raman amplifiers are actually being deployed into systems in commercial service, the practical issues, such as cost, reliability and safety, are yet to be further discussed, particularly for the configuration of distributed amplification. After summarizing the advantages of Raman amplification and reviewing pump laser technologies, this paper will highlight ongoing efforts on practical issues, which include reliability and safety issues of fiber under high-power operations. Finally, it is concluded that by overcoming the above-mentioned practical issues, Raman amplification will stay as a key technology for future optical communications because of its compelling unique advantages

Recorded low power dissipation of 0.14 mW/Gbps in 1060 nm VCSELs for “Green” optical interconnection

Extremely low power dissipation of 0.14 mW/Gbps at 10 Gbps operation with as small as 75 mVp-p of modulation amplitude has been achieved in carefully designed InGaAs/GaAs-MQW 1060 nm VCSELs employed double intra-cavity structure.

Experimental demonstration of low jitter performance and high reliable 1060nm VCSEL arrays for 10Gbpsx12ch optical interconnection

No systematic studies on 1060nm high speed VCSELs have been reported in terms with reliability so far to our best knowledge. In this work, a systematic and intensive study on reliability has been performed for our 1060nm VCSELs consist of double intra-cavity and oxide confined structure with >70ps eye opening margin in Ib=3mA. Estimated power dissipation per bit rate of >5Gbps/mW at Ib=2mA was obtained from low series resistance and low drive voltage characteristics due to effective current injection in our double intra-cavity structure. Aging tests for 3,467pcs discrete non-hermetic VCSELs were performed under 6mA, 70°C to 120°C and up to 5,736 hours, which is equivalent to over 10million device hours in normal operating condition of 40°C and Ib=5mA. We found one degraded device due to the disconnection of the metal interconnecting layer, resulting in 81Fits (C.L.=90%) under Ea=0.35eV and no current accelerated factor. Also, their degradation of threshold current after 1,000 hours operation was less than 0.1mA under high stress condition of >40kA/cm2 and 120°C, which corresponds to more than hundreds year operation. No eye diagram degradation was observed as far as no large threshold current increase under the high stress condition up to 40kA/cm2. It is experimentally proven that inherent potentiality of the VCSELs with 1060nm InGaAs-QW and double intra-cavity structure would be applicable to the future large green data traffic system.

Highly reliable high-power 1480-nm pump lasers for EDFAs and Raman amplifiers

Ultra high power 14XX (1400-1520) nm lasers are required for erbium doped fiber amplifiers (EDFAs) and Raman amplifiers in dense wavelength division multiplexing (DWDM) systems. To realize both high-power and high-reliability performances, we introduced a novel concept in laser diode (LD) chip design in terms of energy conversion efficiency. We also investigated the dependency of laser cavity length on LD characteristics to minimize the total power consumption of both the LD-chip and the LD-module with thermal electric cooler. Using this concept, we successfully reduced the total power consumption of the LD-module by 2Watts, comparing with an old-generation chip design. We have developed the next-generation chips for ultra high power LD-modules with over 300mW fiber-coupled-power in the range of 1400nm-1520nm lasing wavelength, which are suitable pumping sources for Raman amplifiers. The stable operation over 4000hrs in the reliability test under 60 degree C was obtained at 90% of maximum rollover power, indicating that the next-generation chip is robust enough to commercial use. In addition, the cavity length dependency on the reliability performances has been studied. We found that activation energy was 0.62eV, which is consistent with any cavity lengths, and that no degradation of reliability is observed as the facet optical power increases.

Novel concepts in 14XX-nm pump lasers for Raman amplifiers

The requirements for 14XX nm pumps with wavelength range from 1400 to 1520 nm are not only higher output power but also lower degree of polarization (DOP) for preventing the problem due to the polarization-dependent-gain in Raman amplifier. It is expected that co-propagating Raman pumping could improve system performances in comparison with using only counter pumping. To apply this scheme, the pump lasers should have enough low relative intensity noise (RIN) characteristics. In this paper, we will describe two novel concepts in 14XXnm pumps for Raman amplifiers from above requirements. One is a low DOP-laser-module with ultra high optical output, achieving just one beam output with two-orthogonal polarization already scrambled from a SMF pigtail on a single package. The scrambled optical outputs is as high as over 1W and has DOP as low as 5% over the range of operating current. The other is an ultra low noise and wavelength stabilized laser with integrated partial grating in the laser cavity, that is, eliminating an external fiber-Bragg-grating. The laser showed low noise characteristics less than -150dB/Hz of RIN and the stabilized multi-longitudinal-mode oscillation. We also investigated the advantage of this laser in reduction or suppression of the stimulated-Brillouin-scattering induced by pump.

Optical absorption coefficient of carbon-doped GaAs epitaxial layer by means of propagation -loss measurement of waveguide for long wavelength VCSEL

The optical absorption coefficient (α) for carbon-doped GaAs epitaxial layer by CBr<inf>4</inf> with doping range from 10¹⁸ to 10²⁰ cm⁻³ was measured for the first time. Obtained α<inf>GaAs:C</inf> was 2-times larger than α<inf>GaAs:Zn</inf> at 1300nm.

Pump Laser Diodes and WDM Pumping

This chapter discusses issues surrounding the pump laser diodes for broadband Raman amplifiers, which range from fundamentals to industry practices of Raman pump sources based on the so-called 14XX nm pump laser diodes. It also refers to design issues of wavelength-division-multiplexed (WDM) pumping for realizing a broad and flat Raman gain spectrum over the signal band. Section 5.1 introduces fundamentals of pump laser diodes. Section 5.2 refers to the principle and design issues of WDM pumping technique. Section 5.3 discusses details of pump laser diodes and their efficiently combining and depolarizing technologies. Section 5.4 describes practical Raman pump units. And Section 5.5 briefly touches upon ongoing issues on copumped Raman amplifiers and their pumping sources.

Recent progress of high-power 14XX nm pump lasers

High power pump lasers in wavelength range from 1400 to 1520 nm, namely 14XX nm lasers, are heavily demanded for Raman amplification as well as existing Erbium doped fiber (EDF) amplification in recent Dense Wavelength Division Multiplexing (DWDM) optical communication networks.We achieved the record highest optical power output 14XX nm pump laser of 400 mW at the single mode fiber end as a production level. Rollover fiber coupled power was over 500 mW for this device. The pump laser module with newly designed package operated up to the case temperature of 75 degrees C, and laser chip temperature of 25 degrees C. This is the first demonstration at the operating case temperature as high as 75 degree C with 400 mW optical power output range of 14XX nm pump laser module. The technical challenge for higher optical power output operation is how to minimize the heat-generation from the laser chip. For this purpose, we investigated two different design parameters: the operating power consumption and the operating current of the laser.We found that the former parameter is more effective design goal than the latter one, in order to realize both high optical power output operation and long term reliability performances. We also showed that the reliability performance in 14XX nm lasers is not dependent on the facet optical power output, but dominated by the junction temperature at the active region of the lasers.

Enabling VCSEL technology for “Green” optical interconnect in HPC and Data Centers

State of the art VCSEL technology will be reviewed in terms of power consumption and reliability in order to realize the energy efficient HPC and Data Centers. Longer wavelength VCSEL seems promising candidate for the upcoming higher performance computes with energy saving.

High performance “Green” VCSELs for data centers

VCSEL performance will be reviewed. A careful design of cavity structure, as well as precise control in the epitaxial growth enables us to achieve a record power conversion efficiency of 62% in the 1060nm VCSELs.