Mirthless Laughter

Characters don't just laugh when they find something funny. They also tend to laugh in situations when people usually don't. Laughter is an involuntary response and often people don't do it for humor or happiness. Along with laughter, all these types sometimes include crying afterward.As people often cry when important events happen that they cannot control. This can manifest in different ways and convey different meanings:

In Shock: Characters tend to do this when they are reunited with a lost loved one or something they care about deeply. When Stepford Smilers and other characters that have fragile facades do this, it's a sign that their mask is about to break. This type of laughter is usually followed by crying.
Relief: Characters often laugh for relief when a monster avoids them or when long time suffering is put to an end. People living in a deserted island tend to do this when they find food and shelter or when they can finally get off the island. This type of laughter is also sometimes followed by crying.
When nervous: Characters sometimes laugh nervously in response to an upcoming disaster or horrifying event. This often causes other characters to look back at them as if something was wrong with them. Characters also laugh nervously in response to something sorrowful that happened, like a death of a another character and may or may not break down crying.
In fear: People tend to laugh in fear when cornered by a monster or in an Oh Crap situation, while pleading the monster, person or event to stop. This horrified laughter is the least prone to crying afterward. If characters do cry however it's for Tears of Fear. Timid characters and cowards are the ones who tend to laugh in horror. They often tend to scream afterward.

This trope is Truth in Television, as people tend to laugh when extremely nervous or stressed. People might also laugh when they are in physical pain.

Compare Technically a Smile. When smiling is not for happiness or friendliness but is animalistic and predatory in nature. Mirthless Laughter may be the result of a Laugh with Me moment.

Not to be confused with Laughing Mad, which is about mentally insane characters break down laughing.

Examples of Mirthless Laughter include:

Anime and Manga

Jim suffers a bout of nervous laughter, in the midst of a panic attack, during Outlaw Star's 14th episode. Seen at 4:20 here.
Tenchi Muyo: In episode seven, Ryoko and Ayeka join forces to try to win Tenchi's favor by eliminating the competition including each other. They start by tricking Mihoshi into returning to Galaxy Police headquarters, then attempt to seal Washuu in her lab. One by one, however, their plans backfire, which ultimately lands them in the lake outside. And at the exact moment when it seemed things couldn't get worse, Mihoshi returns. The two share a laugh as the last of their schemes quite literally blows up in their faces (10:12-10:49).
During the latter half of The Big O's second season, Angel confesses her feelings for Roger, but he fails to reciprocate. When she asks why, he hesitates, then finally answers that he isn't sure himself; which stuns Angel. Then it hits her. Roger apparently held feelings for R. Dorothy The realization is so jarring for her, it results in bitter laughter. This marks the beginning of her downward spiral for the remainder of the series as her life falls apart.
Higurashi no Naku Koro ni most of the character's laughter is induced by disease-related paranoia.

Film

Luke, Leia, and Han break out into this in Star Wars: A New Hope after R2-D2 manages to shut down the garbage compactor that was about to crush them. They're so raucous that C-3PO first thinks they've actually been crushed.
Poltergeist: After the family searches for Carol Anne in her bedroom they find a small body lying motionless under a bed sheet. Whith much trepidation they pull the sheet only to find it's the creepy clown doll and not their little girl. The mother, Diane, who was on the verge hysterics breaks into laughter, out of a mixture of both nervousness and relief.
Jafar does this uncontrollably in Disney's Aladdin, until Iago snaps him out of it.
In Disney's version of Sleepy Hollow, Ichabod Crane is getting petrified at every sound in the forest - when he finally sees that cattails in the wind are making noises like hoofbeats he starts laughing in frightened relief with a bit of Laughing Mad thrown in.

Literature

Samuel Beckett spends over a page of his novel Watt describing various types of mirthless laughter:

Of all the laughs that strictly speaking are not laughs, but modes of ululation, only three I think need detain us, I mean the bitter, the hollow and the mirthless. They correspond to successive...how shall I say successive...suc...successive excoriations of the understanding, and the passage from the one to the other is the passage from the lesser to the greater, from the lower to the higher, from the outer to the inner, from the gross to the fine, from the matter to the form. The laugh that now is mirthless once was hollow, the laugh that once was hollow once was bitter. And the laugh that once was bitter? Eyewater, Mr. Watt, eyewater. But do not let us waste our time with that. . . . The bitter, the hollow and -- "Haw! Haw!" -- the mirthless. The bitter laugh laughs at that which is not good, it is the ethical laugh. The hollow laugh laughs at that which is not true, it is the intellectual laugh. Not good! Not true! Well well. But the mirthless laugh is the dianoetic laugh, down the snout -- Haw! -- so. It is the laugh of laughs, the risus purus, the laugh laughing at the laugh, the beholding, the saluting of the highest joke, in a word the laugh that laugh -- silence please -- at that which is unhappy.

In the Planet of the Apes novel, Ulysse has this reaction when he definitely realizes the situation he is in (naked, in a cage with animal-like humans, and commanded by armed sapient apes). His laughter also could be the horror version but the shock in being in this position is what most likely caused it.
Sherlock Holmes: a guy who'd had his thumb ripped off that night gets it bandaged up by Watson, and only when it's done does all the stress come back and start laughing. As Watson puts it, "all my medical instincts rose up against that laugh".
Happens once or twice in Animorphs, when characters laugh at a joke more than normal after an adventure because it's "relief laughter. We're-still-alive-laughter".
In the second book of The Dark Elf Trilogy, Dinin breaks into laughter once he realizes that not only is their family under attack and likely about to be wiped out, but also, his sister has just killed his mother to take over power.
In the Belgariad, Mandorallen is stricken by fear for the first time in his life after an encounter with a Wacky Wayside Tribe. Unable to deal with this unfamiliar emotion, which he mistakes for cowardice, he asks his companions for advice on the matter. When someone notes that some people control their fear by laughing at it, he takes the suggestion rather too literally and develops a habit of charging into combat while laughing maniacally.
Rand Al'Thor of The Wheel of Time series does this increasingly as sanity depletes. When pressed, he maintains that he does it to keep from crying, while other characters point out that it would be better for him if he did the latter.

Live Action TV

In Star Trek: The Next Generation, Prior to taking command of the Enterprise-D, Picard laughs after being stabbed in the heart. It's not clear whether he laughed purely out of shock, or if he actually found some humor in it.
Abed in Community does this on occasion but mostly because he doesn't know the proper way to laugh in certain situations.
In Degrassi, Craig bursts into inapropriate laughter at his abusive fathers funeral.
In the first season ender of Buffy the Vampire Slayer, she walks in on Giles and Angel discussing an ancient prophecy that fortells her facing the Master and dying - they're only alerted to her presence as she starts laughing.

Video Games

Yuna and Tidus' infamous fake-laughter scene in Final Fantasy X. They're practicing laughing as a way to de-stress when things get too sad or scary. The result is cringe-worthy.

Web Comics

Schlock Mercenary. The prisoner can hear Schlock.
Chirault, after a magical fight results in the group nearly squashed by a cave-in.

Bethan: Aaron's gone. At... at least... He's had a proper burial? ha... haha...