Diode laser based THz ge­ne­ra­ti­on

The so cal­led 'THz gap' in the range of 100G­Hz to 10THz got its name from the dif­fi­cul­ties to ge­ne­ra­te and de­tect elec­tro­ma­gne­tic ra­dia­ti­on in the fre­quen­cy range. No­nethe­l­ess there is a huge zoo of dif­fe­rent ap­proa­ches and tech­ni­ques to rea­li­ze me­a­su­re­ment sys­tems in this THz gap. The mo­ti­va­ti­on to do so comes from the po­ten­ti­al ap­p­li­ca­ti­ons of the those sys­tems.

When using pho­to­nics con­cepts the sys­tems can be rough­ly ca­te­go­ri­zed by the la­ser­sys­tem which is used:

• Wa­ve­length: The la­ser­sys­tems for THz ge­ne­ra­ti­on are ty­pi­cal­ly eit­her solid state la­sers fiber la­sers, or diode laser. De­pen­ding on the these sys­tems dif­fe­rent wa­ve­length ran­ges for down­con­ver­si­on into the THz range can be ac­ces­sed. In our group we work to­ge­ther with our part­ners with diode la­sers in the ran­ges of 800nm and 1550nm.
• Mo­de­lo­cking: While mo­de­lo­cked la­sers are used for TDS (Time Do­main Spec­trosco­py), con­ti­n­uous wave (CW) la­sers are used for FD (Fre­quen­cy Do­main) sys­tems. Even a Quasi TDS ope­ra­ti­on is pos­si­ble when a high num­ber of un­cor­re­la­ted CW modes are mixed. At our group we work on all three ope­ra­ti­on re­gimes for THz ge­ne­ra­ti­on.
• Re­so­na­tor: Diode la­sers can be used as ac­tive me­di­um in­si­de a laser ca­vi­ty or can be used as mo­no­li­thic laser sour­ces. While an ex­ter­nal ca­vi­ty con­fi­gu­ra­ti­on gives more fle­xi­bi­li­ty, the de­ve­lop­ment of spe­cia­li­sed mo­no­li­thic diode la­sers is the best ap­proach for com­pact and cost ef­fi­ci­ent laser sys­tems.

Here is a short over­view about some of the pho­to­nics THz con­cepts based on se­mi­con­duc­tor la­sers we work on at PTT:

Tunable two-co­lor la­sers for THz ge­ne­ra­ti­on

We have de­ve­lo­ped very fle­xi­ble two-co­lour diode laser sys­tems as sour­ces for our op­to­elec­tro­nic THz sys­tems. Ex­ter­nal ca­vi­ty ar­ran­ge­ments offer the lar­gest fle­xi­bi­li­ty. The Fi­gu­re shows a two-co­lour dou­b­le Lit­trow ar­ran­ge­ment (left) en­ab­ling large tu­ning of the dif­fe­rence fre­quen­cy (right).

The laser out­put was am­pli­fied and cou­p­led into a sim­ple THz trans­mis­si­on sys­tem with a Pho­to­di­ode as THz trans­mit­ter and a Schott­ky bar­ri­er diode for de­tec­tion (see. Fi­gu­re). The use of the sys­tem for sim­ple THz trans­mis­si­on me­a­su­re­ments in the fre­quen­cy range around 0.3 THz was suc­cess­ful­ly pro­ven with hu­mi­di­ty me­a­su­re­ments.

Selec­ted re­fe­ren­ces:

• Y. Hu, B. Khani, C. Bren­ner, V. Ry­ma­nov, A. Stöhr, M. Hof­mann, Two-Co­lor Laser for THz Ge­ne­ra­ti­on with High-Speed Pho­to­di­odes, 2017 Ger­man THz Con­fe­rence, Bo­chum, Ger­ma­ny, March 29-31, 2017.
• Y. Hu, B. Khani, C. Bren­ner, V. Ry­ma­nov, A. Stöhr, M.R. Hof­mann, Com­pact CW THz Spec­trosco­py Sys­tem and its Ap­p­li­ca­ti­on in Water Ab­sorp­ti­on Me­a­su­re­ments, Con­fe­rence on La­sers and Elec­tro Op­tics, CLEO/EU­RO­PE, Mu­nich, Ger­ma­ny, June 2018.

Mul­ti­f­re­quen­cy sys­tems with op­ti­mi­zed THz fre­quen­cy spec­trum

When using more than two op­ti­cal modes for THz ge­ne­ra­ti­on, it is pos­si­ble to ge­ne­ra­te much more THz fre­quen­cy as the num­ber of mi­xing pro­ducts sca­les qua­dra­ti­cal­ly. Un­for­tu­neat­ly the re­sul­ting THz spec­trum is ra­ther in­ho­mo­gen­uous if the op­ti­cal fre­quen­cy spa­cing is not ca­re­ful­ly con­s­i­de­red. An op­ti­mi­zed di­stri­bu­ti­on of THz modes can be achie­ved when the op­ti­cal fre­quen­cy spa­cing is based on a Go­lomb ruler.

For fu­ture in­te­gra­ti­on ex­ter­nal ca­vi­ty laser diode ar­ran­ge­ments are not sui­ta­ble be­cau­se of their com­ple­xi­ty. The­re­fo­re, we have ana­ly­zed mo­no­li­thic two color diode la­sers based on DBR (di­stri­bu­ted Bragg re­flec­tor) and DFB (di­stri­bu­ted feed­back) laser ar­chi­tec­tu­res. This ana­ly­sis star­ted with Y-shaped dou­b­le DBR-la­ser di­odes pro­vi­ded by the Fer­di­nand Braun In­sti­tu­te (FBH). These la­sers show­ed tunable two color ope­ra­ti­on and we suc­cess­ful­ly im­ple­men­ted them into a ho­mo­dy­ne THz sys­tem for ap­p­li­ca­ti­on in THz ab­sorp­ti­on and re­frac­tive index me­a­su­re­ments. Though a proof of prin­ci­ple was suc­cess­ful­ly rea­li­zed, the la­sers pro­vi­ded se­ver­al dif­fi­cul­ties for the de­s­i­red ap­p­li­ca­ti­on in this pro­ject. First, they were de­si­gned for a cen­ter wa­ve­length in the 0.8 µm range which is not com­pa­ti­ble with the trans­mit­ter and re­cei­ver tech­no­lo­gy we use. Se­cond, and more im­portant, the la­sers ex­hi­bit mode hops and im­per­fect tu­ning be­ha­vi­or which in­du­ces major chal­len­ges for THz me­a­su­re­ment ap­p­li­ca­ti­ons.

The­re­fo­re, in the next step, we in­ves­ti­ga­ted Y-shaped dou­b­le DFB two color laser di­odes based on InP tech­no­lo­gy, i.e. emit­ting in the 1.​55 µm wa­ve­length range. The la­sers were pro­vi­ded by Sa­cher La­ser­tech­nik and pro­vi­ded dif­fe­rence fre­quen­cies of around 0.8 THz and 1 THz which was con­ti­n­uous­ly tunable over 50 GHz by va­ria­ti­on of the in­jec­tion cur­rents. The la­sers were im­ple­men­ted into a new THz ho­mo­dy­ne sys­tem for the 1.​55mm range based on com­po­n­ents de­li­ver­ed by our new part­ner at TUDa. In suc­cess­ful pro­of-of-prin­ci­ple stu­dies, we de­mons­tra­ted THz thick­ness me­a­su­re­ments with these de­vices.

Selec­ted re­fe­ren­ces:

• J. O. Gwaro, C. Bren­ner, L.S. Theu­rer, M. Mai­wals, B. Sumpf, and M.R. Hof­mann, “Con­ti­n­uous Wave THz Sys­tem Based on an Elec­tri­cal­ly Tunable Mo­no­li­thic Dual Wa­ve­length Y-Branch DBR Diode Laser, J. of In­fra­red, Mil­li­me­ter and Tera­hertz Waves, Feb. 2020.

THz sys­tems based on mo­de­lo­cked diode la­sers

In ad­di­ti­on to the con­ti­n­uous wave (CW) THz sys­tems di­s­cus­sed above, we have also stu­died mo­de­lo­cked diode laser sys­tems for time do­main spec­trosco­py (TDS). First, we have de­ve­lo­ped a THz TDS sys­tem based on asyn­chro­nous op­ti­cal sam­pling of two ex­ter­nal ca­vi­ty mo­de­lo­cked laser di­odes. The sys­tem setup as shown in the fi­gu­re was suc­cess­ful­ly rea­li­zed and its ope­ra­ti­on was ve­ri­fied but the rea­li­zed band­width was below 0.3 THz due to elec­tro­nic pro­blems and ti­ming jit­ter of the ex­ter­nal ca­vi­ty diode la­sers.[9] The­re­fo­re, we have star­ted to ana­ly­ze dif­fe­rent con­cepts for mo­no­li­ti­cal­ly mo­de­lo­cked diode la­sers that po­ten­ti­al­ly enable in­te­gra­ti­on into our THz sys­tems. The sta­bi­li­ty of these sin­gle-chip de­vices has been and is still ana­ly­zed with re­spect to ap­p­li­ca­ti­ons in THz-TDS sys­tems and as fre­quen­cy combs for CW THz sys­tems.

Ano­ther op­ti­on are so cal­led col­li­ding-pul­se-mo­de­lo­cked laser which can be used for THz ge­ne­ra­ti­on. As in the case of mo­no­li­thic two color la­sers the sys­tem is re­du­ced to a sin­gle chip. The laser emis­si­on is then am­pli­fied and cou­p­led into the THz sys­tem.